Random Sample For Sales Alerts

I'm assuming you have a trigger campaign to send the alert--triggered off of filling the form to request the brief.  First thing to check is that in the Schedule tab you have the campaign set to "Lead can run through campaign once" instead of "every time."  That should stop subsequent alerts after the first goes out, even if the lead download the product brief multiple times.

If you're using mutiple trigger campaigns to send different alerts for each--say you're using different pages & forms and alert messages for different products--one way to limit the alerts is to add leads to a common list when the first alert is sent, then always filter against that list if any subsequent request is made.  I think something like this would work:

Smart List:
1. Trigger (Fills out Form) "Form Name
2. Filter Member of list:  (Not in)  "Already Sent Alert List"

Flow:
1. Send Alert
2. Add to list "Already Sent Alert List"

Schedule: Activate

The first campaign the lead will trigger by filling out a form for Product A, since they're not yet on the Already Sent Alert List, the lead will move to the flow stage.  The second time they fill out a form, the lead will match the trigger (by virtue of filling out the form), but won't pass to the Flow stage becuase they failed to match the filter portion (by virtue of already being in the list).