Should I tell her how I feel if I'm already in a relationship?

I've worked with this girl now for over 6 years, and a few years ago I started having feelings for her. A few years ago, I also got into a relationship with someone outside of work who I still love and want in my life. I know this girl at work likes me to say the least, but I'm dying to know if she loves me/has real feelings for me.

If things weren't complicated, I'd just ask her out on a date. But considering we work together and I'm in a relationship, I feel it would be inappropriate to bring anything up... or would it? Is there anything I can do other than sweep my feelings under the rug?
Should I tell her how I feel if I'm already in a relationship?
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