As DaveSSS says you'll get marginally better performance with an FD made for doubles, but it won't be a world of difference. After all the two larger chainring sizes on most triple setups are the same as they would be with doubles, so the FD's are designed around that.
I suggest that since you already have it (it's free so to speak) you should mount it and see if you're happy with it's performance. If so, used the money you've saved to buy yourself a few beers. Worst case is you'll decide to buy the double after all and will have wasted a bit of time, and of course, no beer.
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