Speak to your tax advisor to understand what the tax burden would be upon sale.
You should know what the tax basis is if you are treating it as a rental. The current depreciation is based on the tax basis.
Regarding section 121 exclusion - There are different rules if you treated it as a personal residence then investment property vs investment property first and then personal residence.
@Sharon Carson You definitely need a CPA’s guidance. A couple of points to be aware of though. —you can’t get the full section 131 exclusion since it has been a rental for 10 years. Any exemption limit, if you moved back into it for 2 years would be prorated between the owner occupied years and the non owner occupied years. As a duplex, this would further to only approximately 1/2 of the pro rated exclusion. —you will have depreciation recapture whether or not you ever took it.
—you’ll have to figure out your cost basis before you can do anything else