diff --git a/src/IPA.Bcfier.App/Controllers/ProjectUsersController.cs b/src/IPA.Bcfier.App/Controllers/ProjectUsersController.cs
index 7ad2ce8..61e5013 100644
--- a/src/IPA.Bcfier.App/Controllers/ProjectUsersController.cs
+++ b/src/IPA.Bcfier.App/Controllers/ProjectUsersController.cs
@@ -78,6 +78,14 @@ public async Task<IActionResult> AddUserToProjectAsync(Guid projectId, [FromBody
                 UserId = dbUser.Id,
                 ProjectId = projectId
             };
+
+            identifierExistsAlready = await _context.ProjectUsers.AnyAsync(pu => pu.ProjectId == projectId
+                && pu.UserId == projectUser.UserId);
+            if (identifierExistsAlready)
+            {
+                return BadRequest(new ApiError("This user assignment already exists."));
+            }
+
             _context.ProjectUsers.Add(projectUser);
             await _context.SaveChangesAsync();
 
diff --git a/src/ipa-bcfier-ui/src/app/components/project-details/project-details.component.ts b/src/ipa-bcfier-ui/src/app/components/project-details/project-details.component.ts
index 091ee7d..7135021 100644
--- a/src/ipa-bcfier-ui/src/app/components/project-details/project-details.component.ts
+++ b/src/ipa-bcfier-ui/src/app/components/project-details/project-details.component.ts
@@ -147,6 +147,9 @@ export class ProjectDetailsComponent implements OnDestroy {
           },
           error: () => {
             this.notificationsService.error('Failed to add the user');
+            this.users$ = this.getProjectUsers(this.data.id);
+            this.filterUsers();
+            this.cdr.detectChanges();
           },
         })
       );