I haven't been able to reliably reproduce this but it seems to happen maybe 1 out of 10 times I enter the build menu of an existing city.

When the bug "occurs", I can build any number of identical improvements in a single town.

I am not using any mods.

I use the monastery here as an example. The bug is not specific to it.

This only works when the city does not have a monastery, yet. When I refresh the Improvement selection window  after building one (by opening/closing it) I can no longer select monasteries to build.

When the bug occurs, the cursor stays in "drop monastery here" mode just like I hadn't built one already. The Improvement selection window does not refresh after building one. The monastery stays highlighted in yellow on the window.

I can then also build multiple different improvements - if I don't close the selection window. I can build 5 monasteries and then 3 town halls.

I did not check if the effects of such buildings stack because I expect that it's going to get fixed anyway. =P