Clear single-frame streak mask if it exists already

Before detecting streaks on the difference warps, clear the existing mask if it
is already there. The streaks detected on the differences should be
better. They are straight, cross multiple detectors, and are not competing
against the background of static sources

python/lsst/drp/tasks/assemble_coadd.py

@@ -1649,6 +1649,12 @@ def findArtifacts(self, templateCoadd, tempExpRefList, imageScalerList):
 
                 if self.config.doFilterMorphological:
                     maskName = self.config.streakMaskName
+                    # clear single frame streak mask if it exists already
+                    try:
+                        warpDiffExp.mask.clearMaskPlane(warpDiffExp.mask.getMaskPlane(maskName))
+                    except pexExceptions.InvalidParameterError:
+                        self.log.debug(f"Did not (need to) clear {maskName} mask because it didn't exist")
+
                     _ = self.maskStreaks.run(warpDiffExp)
                     streakMask = warpDiffExp.mask
                     spanSetStreak = afwGeom.SpanSet.fromMask(