diff --git a/messages/de.msg b/messages/de.msg
index 7209a88d6..9bb712c15 100644
--- a/messages/de.msg
+++ b/messages/de.msg
@@ -99,6 +99,7 @@ UnauthorizedWrite: Sie haben hierfür keine Schreibberechtigung
 EMail: E-Mail
 EMailUnknown email@Text: E-Mail #{email} gehört zu keinem bekannten Benutzer.
 NotAParticipant user@Text tid@TermId csh@Text: #{user} ist nicht im Kurs #{display tid}-#{csh} angemeldet.
+TooManyParticipants: Es wurden zu viele Mitabgebende angegeben
 
 AddCorrector: Zusätzlicher Korrektor
 CorrectorExists user@Text: #{user} ist bereits als Korrektor eingetragen
diff --git a/src/Handler/Submission.hs b/src/Handler/Submission.hs
index 5a73ced6a..aa574d5d6 100644
--- a/src/Handler/Submission.hs
+++ b/src/Handler/Submission.hs
@@ -170,11 +170,11 @@ submissionHelper tid csh shn (SubmissionMode mcid) = do
       (FormFailure failmsgs)   -> return $ FormFailure failmsgs
       (FormSuccess (mFiles,[])) -> return $ FormSuccess (mFiles,[]) -- Type change
       (FormSuccess (mFiles, (map CI.mk -> gEMails@(_:_)))) -- Validate AdHoc Group Members
-        | (Arbitrary {..}) <- sheetGrouping
-        , length gEMails < maxParticipants -> do -- < since submitting user is already accounted for
+        | (Arbitrary {..}) <- sheetGrouping -> do
+          -- , length gEMails < maxParticipants -> do -- < since submitting user is already accounted for
           let gemails = map CI.foldedCase gEMails
               prep :: [(E.Value Text, (E.Value UserId, E.Value Bool, E.Value Bool))] -> Map (CI Text) (Maybe (UserId, Bool, Bool))
-              prep ps = Map.fromList $ map (, Nothing) gEMails ++ [(CI.mk m, Just (i,p,s))|(E.Value m, (E.Value i, E.Value p, E.Value s)) <- ps]
+              prep ps = Map.filter (maybe True $ \(i,_,_) -> i /= uid) . Map.fromList $ map (, Nothing) gEMails ++ [(CI.mk m, Just (i,p,s))|(E.Value m, (E.Value i, E.Value p, E.Value s)) <- ps]
           participants <- fmap prep . E.select . E.from $ \user -> do
             E.where_ $ (E.lower_ $ user E.^. UserEmail) `E.in_` E.valList gemails
             let
@@ -186,20 +186,29 @@ submissionHelper tid csh shn (SubmissionMode mcid) = do
                 E.on $ submissionUser E.^. SubmissionUserSubmission E.==. submission E.^. SubmissionId
                 E.where_ $ submissionUser E.^. SubmissionUserUser E.==. user E.^. UserId
                      E.&&. submission E.^. SubmissionSheet E.==. E.val shid
+                case msmid of -- Multiple `E.where_`-Statements are merged with `&&` in esqueleto 2.5.3
+                  Nothing   -> return ()
+                  Just smid -> E.where_ $ submission E.^. SubmissionId E.!=. E.val smid 
                 return $ E.countRows E.>. E.val (0 :: Int64)
             return (user E.^. UserEmail, (user E.^. UserId, isParticipant, hasSubmitted))
-          $logDebugS "SUBMISSION.AdHocGroupValidation" $ tshow participants
-          mr <- getMessageRender
 
-          let failmsgs = flip Map.foldMapWithKey participants $ \email -> \case
-                  Nothing            -> [mr $ MsgEMailUnknown $ CI.original email]
-                  (Just (_,False,_)) -> [mr $ MsgNotAParticipant (CI.original email) tid csh]
-                  (Just (_,_, True)) -> [mr $ MsgSubmissionAlreadyExistsFor (CI.original email)]
+          $logDebugS "SUBMISSION.AdHocGroupValidation" $ tshow participants
+
+          mr <- getMessageRender
+          let
+            failmsgs = (concat :: [[Text]] -> [Text])
+              [ flip Map.foldMapWithKey participants $ \email -> \case
+                  Nothing            -> pure . mr $ MsgEMailUnknown $ CI.original email
+                  (Just (_,False,_)) -> pure . mr $ MsgNotAParticipant (CI.original email) tid csh
+                  (Just (_,_, True)) -> pure . mr $ MsgSubmissionAlreadyExistsFor (CI.original email)
                   _other             -> mempty
+              , case length participants `compare` maxParticipants of
+                  LT -> mempty
+                  _  -> pure $ mr MsgTooManyParticipants
+              ]
           return $ if null failmsgs
             then FormSuccess (mFiles, foldMap (\(Just (i,_,_)) -> [i]) participants)
             else FormFailure failmsgs
-
         | otherwise -> return $ FormFailure ["Mismatching number of group participants"]