diff --git a/src/main/java/seedu/address/logic/commands/AddCommand.java b/src/main/java/seedu/address/logic/commands/AddCommand.java
index 24707ecf687a..d37e59a42efa 100644
--- a/src/main/java/seedu/address/logic/commands/AddCommand.java
+++ b/src/main/java/seedu/address/logic/commands/AddCommand.java
@@ -17,6 +17,7 @@
 import seedu.address.logic.commands.exceptions.CommandException;
 import seedu.address.model.appointment.Appointment;
 import seedu.address.model.appointment.exceptions.DuplicateAppointmentException;
+import seedu.address.model.appointment.exceptions.DuplicateDateTimeException;
 import seedu.address.model.person.Nric;
 import seedu.address.model.person.Person;
 import seedu.address.model.person.exceptions.DuplicateNricException;
@@ -114,6 +115,7 @@ public class AddCommand extends UndoableCommand {
     public static final String MESSAGE_DUPLICATE_PERSON = "This person already exists in Medeina.";
     public static final String MESSAGE_DUPLICATE_NRIC = "This is already someone with this NRIC.";
     public static final String MESSAGE_DUPLICATE_APPOINTMENT = "This particular appointment already exists in Medeina.";
+    public static final String MESSAGE_DUPLICATE_DATETIME = "This date time is already taken by another appointment.";
     public static final String MESSAGE_DUPLICATE_PET_PATIENT = "This pet patient already exists in Medeina";
     public static final String MESSAGE_INVALID_NRIC = "The specified NRIC does not belong to anyone in Medeina."
             + " Please add a new person.";
@@ -204,6 +206,8 @@ public CommandResult executeUndoableCommand() throws CommandException {
             throw new CommandException(MESSAGE_DUPLICATE_PET_PATIENT);
         } catch (DuplicateAppointmentException e) {
             throw new CommandException(MESSAGE_DUPLICATE_APPOINTMENT);
+        } catch (DuplicateDateTimeException e) {
+            throw new CommandException(MESSAGE_DUPLICATE_DATETIME);
         }
     }
 
@@ -228,7 +232,8 @@ private CommandResult addNewPetPatient() throws DuplicatePetPatientException, Co
     /**
      * Add a new appointment for an existing pet patient under an existing person.
      */
-    private CommandResult addNewAppt() throws CommandException, DuplicateAppointmentException {
+    private CommandResult addNewAppt()
+            throws CommandException, DuplicateAppointmentException, DuplicateDateTimeException {
         Person owner = getPersonWithNric();
         PetPatient pet = getPetPatientWithNricAndName();
         if (owner != null) {
@@ -253,7 +258,7 @@ private CommandResult addNewAppt() throws CommandException, DuplicateAppointment
      * (New appointment for the new patient under a new person).
      */
     private CommandResult addAllNew() throws DuplicatePersonException, DuplicateNricException,
-            DuplicatePetPatientException, DuplicateAppointmentException {
+            DuplicatePetPatientException, DuplicateAppointmentException, DuplicateDateTimeException {
         model.addPerson(toAddOwner);
         model.addPetPatient(toAddPet);
         model.addAppointment(toAddAppt);
diff --git a/src/main/java/seedu/address/model/AddressBook.java b/src/main/java/seedu/address/model/AddressBook.java
index 5115433f0e91..5281abea9602 100644
--- a/src/main/java/seedu/address/model/AddressBook.java
+++ b/src/main/java/seedu/address/model/AddressBook.java
@@ -18,6 +18,7 @@
 import seedu.address.model.appointment.exceptions.AppointmentDependencyNotEmptyException;
 import seedu.address.model.appointment.exceptions.AppointmentNotFoundException;
 import seedu.address.model.appointment.exceptions.DuplicateAppointmentException;
+import seedu.address.model.appointment.exceptions.DuplicateDateTimeException;
 import seedu.address.model.person.Person;
 import seedu.address.model.person.UniquePersonList;
 import seedu.address.model.person.exceptions.DuplicateNricException;
@@ -76,7 +77,8 @@ public void setTags(Set<Tag> tags) {
         this.tags.setTags(tags);
     }
 
-    public void setAppointments(List<Appointment> appointments) throws DuplicateAppointmentException {
+    public void setAppointments(List<Appointment> appointments)
+            throws DuplicateAppointmentException, DuplicateDateTimeException {
         this.appointments.setAppointments(appointments);
     }
 
@@ -109,6 +111,8 @@ public void resetData(ReadOnlyAddressBook newData) {
         try {
             setAppointments(syncedAppointmentList);
         } catch (DuplicateAppointmentException dae) {
+            throw new AssertionError("AddressBook should not have duplicate appointments.");
+        } catch (DuplicateDateTimeException ddte) {
             throw new AssertionError("AddressBook should not have appointments on the same slot");
         }
 
@@ -169,7 +173,7 @@ public void updatePerson(Person target, Person editedPerson)
      *
      * @throws DuplicateAppointmentException if an equivalent person already exists.
      */
-    public void addAppointment(Appointment a) throws DuplicateAppointmentException {
+    public void addAppointment(Appointment a) throws DuplicateAppointmentException, DuplicateDateTimeException {
         Appointment appointment = syncWithAppointmentMasterTagList(a);
         // TODO: the tags master list will be updated even though the below line fails.
         // This can cause the tags master list to have additional tags that are not tagged to any appointment
diff --git a/src/main/java/seedu/address/model/Model.java b/src/main/java/seedu/address/model/Model.java
index 1dbb644240ad..47a635b68cbd 100644
--- a/src/main/java/seedu/address/model/Model.java
+++ b/src/main/java/seedu/address/model/Model.java
@@ -7,6 +7,7 @@
 import seedu.address.model.appointment.exceptions.AppointmentDependencyNotEmptyException;
 import seedu.address.model.appointment.exceptions.AppointmentNotFoundException;
 import seedu.address.model.appointment.exceptions.DuplicateAppointmentException;
+import seedu.address.model.appointment.exceptions.DuplicateDateTimeException;
 import seedu.address.model.person.Person;
 import seedu.address.model.person.exceptions.DuplicateNricException;
 import seedu.address.model.person.exceptions.DuplicatePersonException;
@@ -56,7 +57,7 @@ void updatePerson(Person target, Person editedPerson)
     void deleteTag(Tag tag);
 
     /** Adds the given appointment */
-    void addAppointment(Appointment appointment) throws DuplicateAppointmentException;
+    void addAppointment(Appointment appointment) throws DuplicateAppointmentException, DuplicateDateTimeException;
 
     /** Deletes the given appointment. */
     void deleteAppointment(Appointment target) throws AppointmentNotFoundException;
diff --git a/src/main/java/seedu/address/model/ModelManager.java b/src/main/java/seedu/address/model/ModelManager.java
index a8d45b51ac71..12e482bab0d5 100644
--- a/src/main/java/seedu/address/model/ModelManager.java
+++ b/src/main/java/seedu/address/model/ModelManager.java
@@ -16,6 +16,7 @@
 import seedu.address.model.appointment.exceptions.AppointmentDependencyNotEmptyException;
 import seedu.address.model.appointment.exceptions.AppointmentNotFoundException;
 import seedu.address.model.appointment.exceptions.DuplicateAppointmentException;
+import seedu.address.model.appointment.exceptions.DuplicateDateTimeException;
 import seedu.address.model.person.Person;
 import seedu.address.model.person.exceptions.DuplicateNricException;
 import seedu.address.model.person.exceptions.DuplicatePersonException;
@@ -131,7 +132,8 @@ public synchronized void deleteAppointment(Appointment target) throws Appointmen
     }
 
     @Override
-    public synchronized void addAppointment(Appointment appointment) throws DuplicateAppointmentException {
+    public synchronized void addAppointment(Appointment appointment)
+            throws DuplicateAppointmentException, DuplicateDateTimeException {
         addressBook.addAppointment(appointment);
         updateFilteredAppointmentList(PREDICATE_SHOW_ALL_APPOINTMENTS);
         indicateAddressBookChanged();
diff --git a/src/main/java/seedu/address/model/appointment/UniqueAppointmentList.java b/src/main/java/seedu/address/model/appointment/UniqueAppointmentList.java
index 846781f911b3..95cc113bfedf 100644
--- a/src/main/java/seedu/address/model/appointment/UniqueAppointmentList.java
+++ b/src/main/java/seedu/address/model/appointment/UniqueAppointmentList.java
@@ -10,6 +10,7 @@
 import javafx.collections.ObservableList;
 import seedu.address.model.appointment.exceptions.AppointmentNotFoundException;
 import seedu.address.model.appointment.exceptions.DuplicateAppointmentException;
+import seedu.address.model.appointment.exceptions.DuplicateDateTimeException;
 
 /**
  * A list of appointments that enforces uniqueness between its elements and does not allow nulls.
@@ -35,11 +36,17 @@ public boolean contains(Appointment toCheck) {
      *
      * @throws DuplicateAppointmentException if the person to add is a duplicate of an existing person in the list.
      */
-    public void add(Appointment toAdd) throws DuplicateAppointmentException {
+    public void add(Appointment toAdd) throws DuplicateAppointmentException, DuplicateDateTimeException {
         requireNonNull(toAdd);
         if (contains(toAdd)) {
             throw new DuplicateAppointmentException();
         }
+
+        for (Appointment a : internalList) {
+            if (a.getDateTime().equals(toAdd.getDateTime())) {
+                throw new DuplicateDateTimeException();
+            }
+        }
         internalList.add(toAdd);
     }
 
@@ -84,7 +91,8 @@ public void setAppointments(UniqueAppointmentList replacement) {
         this.internalList.setAll(replacement.internalList);
     }
 
-    public void setAppointments(List<Appointment> appointments) throws DuplicateAppointmentException {
+    public void setAppointments(List<Appointment> appointments)
+            throws DuplicateAppointmentException, DuplicateDateTimeException {
         requireAllNonNull(appointments);
         final UniqueAppointmentList replacement = new UniqueAppointmentList();
         for (final Appointment appointment : appointments) {
diff --git a/src/main/java/seedu/address/model/appointment/exceptions/DuplicateAppointmentException.java b/src/main/java/seedu/address/model/appointment/exceptions/DuplicateAppointmentException.java
index c224529c211a..89890ced67b2 100644
--- a/src/main/java/seedu/address/model/appointment/exceptions/DuplicateAppointmentException.java
+++ b/src/main/java/seedu/address/model/appointment/exceptions/DuplicateAppointmentException.java
@@ -7,6 +7,6 @@
  */
 public class DuplicateAppointmentException extends DuplicateDataException {
     public DuplicateAppointmentException() {
-        super("Operation would result in duplicate persons");
+        super("Operation would result in duplicate appointments");
     }
 }
diff --git a/src/main/java/seedu/address/model/appointment/exceptions/DuplicateDateTimeException.java b/src/main/java/seedu/address/model/appointment/exceptions/DuplicateDateTimeException.java
new file mode 100644
index 000000000000..71e79a3beb71
--- /dev/null
+++ b/src/main/java/seedu/address/model/appointment/exceptions/DuplicateDateTimeException.java
@@ -0,0 +1,12 @@
+package seedu.address.model.appointment.exceptions;
+
+import seedu.address.commons.exceptions.DuplicateDataException;
+
+/**
+ * Signals that the operation will result in double booking.
+ */
+public class DuplicateDateTimeException extends DuplicateDataException {
+    public DuplicateDateTimeException() {
+        super("Operation would result in multiple bookings in the same time slot");
+    }
+}