What type of setup project have you used?
If you want your installer/setup to overwrite existing installation then you must increase the version number, every time you build installer project.
Version number consists of 4 parts 0.0.0.0 (Major Version, Minor Version, Build Number, Revision Number) but windows installer consider only first 3 parts so, make sure to increase the build number, every time you build a new installer.
If you run the same installer after installation was already completed successfully then it will ask you to remove the installation but if the installer is new but with same version number (you rebuild the installer project but did not change the version number) then you will get the in question message.
Select setup project in Visual studio > F4 > change the Version number (increase the build number by 1)