SlickEdit will only select one C/C++ compiler configuration, depending on your project settings or the default compiler setting. However, if what you really have is a C/C++ compiler configuration (probably the G++ cygwin which is auto-detected), and another Language specific tag file for the ARM compiler, then both would be selected.
In this case, you would need to remove the Language specific tag file and create a specific compiler configuration for the ARM compiler (Document > C/C++ Options > Compiler Properties > New..., and point it at the directories for the ARM compiler. Then you will need to make sure that your projects point to the ARM compiler rather than the cygwin compiler.
You can also go to Tools > Tag Files and look at what source files are tagged for each of these compilers to be certain that there isn't overlap. That would be another reason you could be seeing both symbols from the ARM compiler and the Cygwin compiler.