Error in 401K rollover to new platform. Same employer

My Company changed their 401K platform due to a partner split. Prior to this change I took out my portion and rolled it into an IRA. Two months later when they rolled all other employees amounts over to the new platform they had the numbers wrong and added my previous amounts to it.

If they made an error in your account they probably also made an error in somebody else's account, otherwise the total value wouldn't have matched.

What are my legal options here?

Contact your company and the old and new platform. Politely demand that they resolve the issue, without you having to move funds.

They want me to roll it back over to the previous partners account. Will I have penalties doing this? This was their error.

This should not cost you any money to resolve. The corrective actions should not impact your taxes, or you ability to contribute the amounts you want into your 401(k) or IRA. If they propose any action that would cost you money or have a tax impact, demand a better solution.