Can I reuse an insulin syringe for a second dose later in the day?

Don't. It's false economy. Insulin needles are like 15 cents each in bulk. You want a sterile needle and a sharp tip. A reused needle is duller (more painful) and contamination risk is nonzero, especially if you're pulling from a multi-dose vial repeatedly.

Used needles on second stick hurt like hell. That alone is reason enough.

Also a sterile needle each time means if you get a bad injection site, you know it wasn't the needle.

Ok fair, pitching them.

saving on syringes but spending $80 on peptides is backwards economy.

Box of 100 BD insulin syringes is like $15. There's no good reason to reuse.

Honestly if you're trying to dial in how a peptide actually works on you, reusing a needle is just adding a variable you don't need. Dull needle changes injection depth, angle gets weird, and you can't isolate whether something didn't work because the peptide sucked or because your second injection of the day was sloppy. Cost per syringe is basically nothing. Just grab a fresh one.