Will you be labeled a warmonger if keep recapturing an enemy city?

Quote from the wiki : about warmonging points

These points are only applied if you capture a city that was not founded by you

So as you're not the original owner you will suffer multiple penalties. However the larger the map is, the less the penalty is (see the wiki for the math calculation).

Actually, yes. You will receive a penalty each time a city you did not found is captured by you. It doesn't matter if it was retaken in the last turn. You capture a city = You gain warmonging points.

If you want to avoid being a warmonger, clear out the enemy units first and surround the city as fast as possible in order to protect it from melee units which are the only one able to capture a city.