diff --git a/src/locales/en/messages.json b/src/locales/en/messages.json
index 34264765..11e555bc 100644
--- a/src/locales/en/messages.json
+++ b/src/locales/en/messages.json
@@ -441,6 +441,19 @@
     "sync": {
         "message": "Sync"
     },
+    "duplicateEmails": {
+        "message": "Emails must be unique. Multiple entries pulled with the following emails:",
+        "desription": "Error message displayed when duplicate email addresses are synced. Followed by a list of duplicate emails."
+    },
+    "andMore": {
+        "message": "and $NUMBER$ more...",
+        "placeholders": {
+            "NUMBER": {
+                "content": "$1",
+                "example": "10"
+            }
+        }
+    },
     "ldapEncrypted": {
         "message": "This server uses an encrypted connection"
     },
diff --git a/src/services/sync.service.ts b/src/services/sync.service.ts
index 10cbda53..ce59e880 100644
--- a/src/services/sync.service.ts
+++ b/src/services/sync.service.ts
@@ -55,6 +55,14 @@ export class SyncService {
                 this.flattenUsersToGroups(groups, groups);
             }
 
+            const duplicateEmails = this.findDuplicateUserEmails(users);
+            if (duplicateEmails.length > 0) {
+                const emailsMessage = duplicateEmails.length < 4 ?
+                    duplicateEmails.join('\n') :
+                    duplicateEmails.slice(0, 3).join('\n') + '\n' + this.i18nService.t('andMore', `${duplicateEmails.length - 3}`);
+                throw new Error(this.i18nService.t('duplicateEmails') + '\n' + emailsMessage);
+            }
+
             if (test || (!syncConfig.overwriteExisting &&
                 (groups == null || groups.length === 0) && (users == null || users.length === 0))) {
                 if (!test) {
@@ -108,6 +116,19 @@ export class SyncService {
         }
     }
 
+    private findDuplicateUserEmails(users: UserEntry[]) {
+        const duplicatedEmails = new Array<string>();
+        users.reduce((agg, user) => {
+            if (agg.includes(user.email) && !duplicatedEmails.includes(user.email)) {
+                duplicatedEmails.push(user.email);
+            } else {
+                agg.push(user.email);
+            }
+            return agg;
+        }, new Array<string>());
+        return duplicatedEmails;
+    }
+
     private filterUnsupportedUsers(users: UserEntry[]): UserEntry[] {
         return users == null ? null : users.filter(u => u.email?.length <= 256);
     }