Is there any benefit (or detriment) to gelatin fining a beer twice?

If you have already gelatin fined your brew once, then it may be that the haze is not being caused by yeast in suspension, which gelatin is intended to tackle.

A possible alternative is that your beer is hazy due to long proteins in suspension and a different fining agent is required to tackle this issue. Polyclar is one such agent and can be added after fermentation in primary / secondary.

It's actually hard to say without knowing a few things. Did you use all barley? Wheat will make your beer cloudy. Did you dry hop with pellets? If so, chill your secondary fermentor before you rack your beer over to kegs or bottles.

There's certainly no harm in trying it again. Any gelatin you add will sink to the bottom regardless of whether or not it takes any haze-causing particles with it. In my experience, different beers have vastly different requirements for fining, some needing several times as much fining as an easy-to-fine one.

Some things about IPAs and haze:

• Hops, whether added in the boil or as dry-hops, proportionally increase the polyphenol content of wort and beer and, depending on the beer's protein level, may contribute to haze.
• A dry-hopped IPA probably has a comparatively higher lipid content (from aromatic hop oils) and this may contribute a light haze in the form of a colloidal suspension of the oil.

That being said, I'm not particularly sure from my own experience how well gelatin will work in these specific cases, but I don't see the downside in trying.