How to keep myself away from him?

I have a friend who is my coworker. Earlier he told me that he found me attractive and maybe he loved me. But I'm married. We didn't talk to each other for 9 months. We just worked. He talked to me again last month. He said again today that he is still in love with me. I told him we could just be friends. I will not be unfaithful. He was sad and upset How to deal with him at work?
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I thought we could be friends. I didn't know about his feelings.
How to keep myself away from him?
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