It seems as if you may have learned something from this game. I'm no expert, but here are a couple of comments.
1. I wouldn't have taken the knight on move 5. Your bishop was reasonably well-placed where it was so there was no overriding reason to trade it off.
2. Doubled pawns can indeed be a weakness, but this fact isn't an iron-clad rule. In exchange for this doubled pawn, your opponent gave himself access to a half-open e-file and control over the e5 square.
3. I probably would have taken the f-pawn on move 7. Then, if he had retaken it, his king would have been a little exposed. And the f-pawns would not only have been doubled, they would have been isolated. Your king would have been temporarily exposed, but you were going to castle soon anyway, so this probably wouldn't have been a big deal.
4. And, of course, if anyone has told you not to look a gift horse in the mouth, ignore them. In chess, it is wise to carefully examine any "gift" your opponent gives you. It may indeed be a blunder, but make double plus sure about that before taking any piece. But I'm pretty sure you have already realized this.
Well, this is a recent game I played one, I was up in material, and got very sloppy.
Please tell me what I should have done, especially how to deal with his F pawn early in the game.