Form-handling code executing twice

I wrote a plugin whose form-handling code is being executed twice: once before the page header is output and again when the page’s main content is being produced. It should only execute when the main content is produced. (I suspect that my theme is improperly causing it to execute when the header is being generated.)

Is there a theme tag to test whether code is being executed during content-generation time? That would give me an easy fix.