diff --git a/api/system/info.py b/api/system/info.py
index 89eec38f..32729298 100644
--- a/api/system/info.py
+++ b/api/system/info.py
@@ -162,7 +162,7 @@ async def get_user_sites(
             # record matches the current site
             current_site_exists = True
             if user_name not in site.users:
-                current_site.users.extend(site.users)
+                current_site.users.update(site.users)
                 current_needs_update = True
             # we can use elif here, because we only need to check one condition
             elif site.platform != current_site.platform:
diff --git a/api/system/sites.py b/api/system/sites.py
index e4d37c29..7b23a4e7 100644
--- a/api/system/sites.py
+++ b/api/system/sites.py
@@ -12,7 +12,7 @@ class SiteInfo(OPModel):
     platform: Platform = Field(...)
     hostname: str = Field(..., title="Machine hostname")
     version: str = Field(..., title="Ayon version")
-    users: list[str] = Field(..., title="List of users")
+    users: set[str] = Field(..., title="List of users")
 
 
 @router.get("/system/sites", tags=["System"])
diff --git a/ayon_server/utils.py b/ayon_server/utils.py
index ad312d3d..104b47a1 100644
--- a/ayon_server/utils.py
+++ b/ayon_server/utils.py
@@ -49,6 +49,9 @@ def json_default_handler(value: Any) -> Any:
     if isinstance(value, datetime.datetime):
         return value.isoformat()
 
+    if isinstance(value, set):
+        return list(value)
+
     raise TypeError(f"Type {type(value)} is not JSON serializable")