diff --git a/webservice/messages.py b/webservice/messages.py
index 4928545df757c3e8560e51eb714de4aca2ebdfb2..48b17b367cc4465c365fa129edf260c7d80ee1bb 100644
--- a/webservice/messages.py
+++ b/webservice/messages.py
@@ -43,3 +43,4 @@ class Success:
     REPROD_QUEUED = "SUCCESS: Queued proposal {}, run {} for reproducing previous offline calibration"
     DONE_CORRECTION = "SUCCESS: Finished correction: proposal {}. run {}"
     DONE_CHAR = "SUCCESS: Finished dark characterization: proposal {}, run {}"
+    ALREADY_REQUESTED = "SUCCESS: Correction already queued/running for proposal {}, run {}"
diff --git a/webservice/webservice.py b/webservice/webservice.py
index 45ec4ecf9d0bfc496244ad0ea0dda65068c9e503..2663b27f416e6db59aff20cb3c95cd9734528c00 100644
--- a/webservice/webservice.py
+++ b/webservice/webservice.py
@@ -62,6 +62,7 @@ def init_job_db(config):
             action,
             timestamp
         );
+        CREATE INDEX IF NOT EXISTS req_by_run ON requests(proposal, run, action);
         CREATE TABLE IF NOT EXISTS executions(
             exec_id INTEGER PRIMARY KEY,
             req_id REFERENCES requests(req_id),
@@ -962,6 +963,12 @@ class ActionsServer:
             proposal = self._normalise_proposal_num(proposal)
             pconf_full = self.load_proposal_config(cycle, proposal)
 
+            if self.check_unfinished_correction(proposal, int(runnr)):
+                # A correction is already running for this run
+                msg = Success.ALREADY_REQUESTED.format(proposal, runnr)
+                logging.debug(msg)
+                return msg.encode()
+
             with time_db_transaction(self.job_db, 'Insert request'):
                 cur = self.job_db.execute(  # 2
                     "INSERT INTO requests VALUES (NULL, ?, ?, ?, 'CORRECT', ?)",
@@ -1082,6 +1089,12 @@ class ActionsServer:
         request_time = datetime.now()
 
         try:
+            if self.check_unfinished_correction(proposal, int(runnr)):
+                # A correction is already running for this run
+                msg = Success.ALREADY_REQUESTED.format(proposal, runnr)
+                logging.debug(msg)
+                return msg.encode()
+
             with time_db_transaction(self.job_db, 'Insert request'):
                 cur = self.job_db.execute(
                     "INSERT INTO requests VALUES (NULL, ?, ?, ?, 'CORRECT', ?)",
@@ -1368,6 +1381,16 @@ class ActionsServer:
 
     # Helper methods for handlers ---------------------------------------------
 
+    def check_unfinished_correction(self, proposal: str, runnr: int):
+        row = self.job_db.execute(
+            "SELECT job_id FROM slurm_jobs "
+            "INNER JOIN executions USING (exec_id) "
+            "INNER JOIN requests USING (req_id) "
+            "WHERE proposal = ? AND run = ? AND action = 'CORRECT' "
+            "  AND slurm_jobs.finished = 0", (proposal, runnr)
+        ).fetchone()
+        return row is not None
+
     @staticmethod
     def _normalise_proposal_num(p: str) -> str:
         return "{:06d}".format(int(p.strip('p')))