It sure is. It was quite a pain in the past to try to keep users from hitting the back button. It now works in WebDirect. Do note, however, this from the web direct guide.
If a web user clicks the web browser's Reload or Refresh button, FileMaker
WebDirect leaves the current session without logging out or activating the OnWindowClose and
OnLastWindowClose script triggers. It then creates a new connection to FileMaker
Server, activating the OnFirstWindowOpen and OnWindowOpen script triggers
So, hitting refresh/reload could cause a startup script to rbe run again, potentially producing undesired results, if I'm reading that correctly.
Guide can be found here: https://fmhelp.filemaker.com/docs/13/en/fm13_webdirect_guide.pdf
EDIT: ACCIDENTATLLY COPY/PASTED WRONG PARAGRAPH FROM GUIDE