There is a bug in the module installer. When installing 3'rd party modules, if a module is installed that has the same class_prefix as a module that is currently installed, the installer does not return an error. Instead it says it was succesful installed, but still remains in the uninstalled list yet will execute the install SQL.
So the module is technically installed but does not show up in the sys_modules table as installed due to the error that occurs when the installer tries to create that entry.
Ran into this incompatibility problem between my FAQ editor and Faq version 1.0 by Mods4Dolphin Both are using the same class_prefix.
I have not tested if this problem occurs if the db prefix is the same, but most likely does.
In any case, the module installer should detect this problem and fail the install.