Schrobby said: IQDB doesn't find them all. Pictures already uploaded from Seiga don't show up when you check a bit identical one from pixiv.
The only reason this should happen is if the existing duplicate was uploaded too recently for IQDB to have updated. IQDB does work on visual similarity, so a bit-identical image in its database will always match (as will resized versions and minor revisions). Crops, rotations, and major color changes do seem to sometimes trip it up though.
I actually do the opposite, I check IQDB every time to make sure it's not a lesser duplicate first (not everything is tagged with it's artist, there's still a fair number of posts from image aggregators and nonpixiv sources where an artist can't or hasn't yet been identified). As Kikimaru states, it's quite tedious to search the artist every time. For me this method seems faster. I may feel this way because I have an extension for IQDB, but don't have an (the?) script to auto check artists.
I guess it tends to work for me, as I'm rarely posting hours-old content. In the few cases where I end up uploading a duplicate (usually due to an IQDB crop issue or other edit), and it's not good enough to keep as such, I just delete it myself.