Is it proper for a kohein to disqualify himself from duchaning if he feels that he is ritually unpure at that particular moment?
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If you mean technical ritual impurity (tum'ah) - then that's not an issue; all Jews nowadays are presumed tamei (and if they live in the Diaspora, then they are impure anyway, since all places outside of Eretz Yisrael are tamei by Rabbinical decree).
If you mean that he feels unworthy - that's not a good reason to not perform birkas kohanim; that would amount to saying that because he's committed one sin, he should commit another. Ultimately, whether a particular kohen is worthy or not doesn't matter (aside from certain sins): the blessing comes from Hashem, not from the kohen. (Shulchan Aruch Harav 128:52 citing Rambam)