one of my FR got flat out, so I bought a pair of front used tires ~about 5k miles on them. (got a deal on pair of them)

and my original tires are also around 5k miles. so should I replace both front or just replace FR?

 

I probably would get both replaced and use the good one you had as a spare. But that's just me.

 

I have replaced one tire because of nail damage. Same brand. No problem.