is it normal to repeat the same settler quests over again ?, i've now been to the corvega assembly plant numerous times to kill raiders & yet they still re appear even with the ' cleared ' sign at the bottom of the location

I would not be surprised; most of those Radiant quests are pretty much randomly generated (although they DO seem to re-use many buildings, like Hardware Town near Diamond City)... the good news, sort of, is that for the settler quest there is usually only really one main foe to defeat to "clear" the area. Last night I entered an interior area, killed one (named) NPC and the quest finished, even though I know there were a ton of other raiders in the area.

Good ol' http://elderscrolls.wikia.com/wiki/Radiant_Quests in Bethesda style. No, I didn't screw up the URL. Keywords for further reading: Skyrim has infinite quests.

i've been sent back to BADFTL several times tho one was to rescue a hostage and the second time was to clear the place out.

i've never been sent back to the same place for the same reason twice. tho the settlers love telling me how inept they are at defending against 3 raiders