If an employer paid me too much am I obligated to pay it back even though it was their mistake and 5 months ago?

Question Details: 5 months ago, my employer direct deposited an extra paycheck after I resigned. They told me that the check was cancelled and the money would be deducted from my account. I did not keep track of the money but I guess they never deducted it. Now, 5 months later they are telling me that I owe them over $900, which I do NOT have. Am I obligated to pay them back? If so, do I have to pay the full amount? Especially since it was their mistake and over 5 months ago! Thanks!

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