In an attempt to pay back their student loans a bit faster, mary and bob commit identity theft and impersonate others so they can use their credit cards. the government prosecutes them but loses due to lack of evidence. after more evidence surfaces, the government attempts to prosecute mary and bob again. in doing so, it may be violating which amendment?

The government may be violating the Fifth Amendment by trying to prosecute Mary and Bob again.
The Fifth Amendment states (among other things) that a person can be tried only once in federal court for the same offense. Given that Mary and Bob were already prosecuted for identity theft and won the case due to lack of evidence, the government can try to prosecute them again, but it will be violating the Fifth Amendment in doing so.