if you dont give a rematch in a close fight is it considered ducking??

You should have a third choice for "it depends." I don't think it is if the next fight is every bit as dangerous as the one in which the rematch wasn't given.
Two fights for instance DLH/Quartey and Mosley/Cotto. Both deserved rematches but we can look the other way when we know DLH/Quartey was precursor to Trinidad. Mosley/Cotto needs a rematch but Margarito is a very tough fight too. Pac/Marquez 2 is vindicacted by Pac having had his eyes on moving up in weight for some time. Boxers fight to advance level by level, sometimes they can't stop just to settle scores for good as in the old days.