I've faced this problem when posting updates of my plugin. There seems to be a bit of unlikely behavior of SM when a plugin is overwritten many times.
In my case, I tend to have mistakes on my plugins and try to correct them only to find out later that I missed a few and I try correcting them again over and over until all mistakes have been corrected. You'll get the idea.
The only solution I found is renaming the .sp file, compiling it and putting the compiled .smx in the plugins folder, and deleting any traces of the old one. Why don't you try it? I hope this helps.
Edit: Or.... this could be the case where two versions of the plugin existing with different names.