You can file at any time, BUT when you get the W-2, you will HAVE to file an amended return. This can cost you and is a hassle.
Amened returns take much longer to process, so if you get a refund from your amended return, you are likely to have to wait 12-16 weeks. Probably better to wait for your next W-2.
If your employer sent a copy of your W-2 to the IRS (like they were supposed to), you can get a copy (data only) for free from the IRS at Get Transcript.
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