I got this same error after setting up yet another local test gallery. I install them all on the same database, I just use different table prefixes.
So for example, I have one instance of zenphoto installed at [b]/one[/b] then my table names would be [b]one_albums[/b], [b]one_images[/b], [b]one_comments[/b].
Before I got this error I only had 2 installations, one with zenphoto 1.0.2 another with zenphoto 1.0.3. Now that I'm installing a second version of 1.0.3 I get this error. According to THIS there's a conflict with the contraints.
May I suggest in the next version that the constraint names are also prefixed with the table prefix?
I've modified the setup.php to do this and I'm now able to install the second instance of 1.0.3.