fix(cron): disallow jobs executing twice within scheduling precision

src/Cron.hs

@@ -155,7 +155,7 @@ nextCronMatch :: TZ -- ^ Timezone of the `Cron`-Entry
               -> UTCTime -- ^ Current time, used only for `CronCalendar`
               -> Cron
               -> CronNextMatch UTCTime
-nextCronMatch tz mPrev prec now c@Cron{..} = case notAfter of
+nextCronMatch tz mPrev prec now c@Cron{..} = onlyOnceWithinPrec $ case notAfter of
   MatchAsap -> MatchNone
   MatchAt ts
     | MatchAt ts' <- nextMatch
@@ -165,6 +165,16 @@ nextCronMatch tz mPrev prec now c@Cron{..} = case notAfter of
     | otherwise -> MatchNone
   MatchNone -> nextMatch
   where
+    onlyOnceWithinPrec sched = case mPrev of
+      Nothing -> sched
+      Just prevT -> case sched of
+        MatchAsap
+          | now >= addUTCTime prec prevT -> MatchAsap
+          | otherwise -> MatchAt $ addUTCTime prec prevT
+        MatchAt ts -> let ts' = max ts $ addUTCTime prec prevT
+                       in if | ts' <= addUTCTime prec now -> MatchAsap
+                             | otherwise -> MatchAt ts'
+        MatchNone  -> MatchNone
     notAfter
       | Right c' <- cronNotAfter
       , Just ref <- notAfterRef