Assuming (which is very safe in most cases) that the employment contract has something in it that is being violated by double-dipping, it is both garden variety fraud (in many cases state and therefore dependent on the state laws in question), and in some cases federal depending on the circumstances. In the jurisdictions where fraud against an employer is based on the amount of money at play, it's still almost always going to be a felony because it takes very little time to reach the common thresholds (e.g. $1k) when double-dipping.
Additionally, it's a pretty obvious opening to a lawsuit in at least most jurisdictions for breach of fiduciary duty (under the duty of loyalty -- Google this + your own jurisdiction for more details on what it covers where you are) and breach of contract.
Now, the employer probably won't sue or prosecute in most circumstances, so you may get away with it. But that's some high-stakes roulette, especially since you're not going to get UI if you get fired for gross misconduct and you're not going to get a good recommendation.
Additionally, it's a pretty obvious opening to a lawsuit in at least most jurisdictions for breach of fiduciary duty (under the duty of loyalty -- Google this + your own jurisdiction for more details on what it covers where you are) and breach of contract.
Now, the employer probably won't sue or prosecute in most circumstances, so you may get away with it. But that's some high-stakes roulette, especially since you're not going to get UI if you get fired for gross misconduct and you're not going to get a good recommendation.