diff --git a/modules/api/php/endpoints/candidates.class.inc b/modules/api/php/endpoints/candidates.class.inc
index 64ae7d6c7e2..ed56e9a7242 100644
--- a/modules/api/php/endpoints/candidates.class.inc
+++ b/modules/api/php/endpoints/candidates.class.inc
@@ -241,8 +241,14 @@ class Candidates extends Endpoint implements \LORIS\Middleware\ETagCalculator
                 $pscid,
                 $project->getId()
             );
-        } catch (\LorisException | \InvalidArgumentException $e) {
-            return new \LORIS\Http\Response\JSON\BadRequest($e->getMessage());
+        } catch (\ConflictException $e) {
+            return new \LORIS\Http\Response\JSON\Conflict(
+                $e->getMessage()
+            );
+        } catch (\Exception | \LorisException | \InvalidArgumentException $e) {
+            return new \LORIS\Http\Response\JSON\BadRequest(
+                $e->getMessage()
+            );
         }
 
         $candidate = \NDB_Factory::singleton()->candidate($candid);
diff --git a/php/exceptions/ConflictException.class.inc b/php/exceptions/ConflictException.class.inc
new file mode 100644
index 00000000000..0c5ef1b0bfa
--- /dev/null
+++ b/php/exceptions/ConflictException.class.inc
@@ -0,0 +1,11 @@
+<?php
+/**
+ * This file contains the Conflict exception type.
+ *
+ * PHP Version 7
+ *
+ * @license http://www.gnu.org/licenses/gpl-3.0.txt GPLv3
+ */
+class ConflictException extends LorisException
+{
+}
diff --git a/php/libraries/Candidate.class.inc b/php/libraries/Candidate.class.inc
index cf987cf047b..cf71013f0e9 100644
--- a/php/libraries/Candidate.class.inc
+++ b/php/libraries/Candidate.class.inc
@@ -223,6 +223,7 @@ class Candidate implements \LORIS\StudyEntities\AccessibleResource,
         ProjectID $registrationProjectID = null
     ): CandID {
         $factory = NDB_Factory::singleton();
+        $db      = $factory->database();
 
         $site = \Site::singleton($centerID);
 
@@ -264,6 +265,25 @@ class Candidate implements \LORIS\StudyEntities\AccessibleResource,
                 );
             }
 
+            // check pscid uniqueness
+            $existing = $db->pselectOne(
+                'SELECT
+                  COUNT(*)
+                 FROM candidate
+                 WHERE PSCID = :v_pscid
+                 GROUP BY
+                   PSCID
+                ',
+                ['v_pscid' => $PSCID]
+            );
+
+            if ($existing > 0) {
+                throw new \ConflictException(
+                    "PSCID must be unique",
+                    PSCID_NOT_UNIQUE
+                );
+            }
+
             // check pscid structure
             if (!Candidate::validatePSCID(
                 $PSCID,