Time based workflows that takes into consideration the changing of a date

Hi

I have a requirement based on an end date that is manually updated in our CRM.

I need to send a reminder email a month before the end date - which i have figured how to do both on a modified and new record and the rule condition being:

IfNull(PRE('<dMaintenance_End_Date_1_ITAG>'),'NULL') <> [<dMaintenance_End_Date_1_ITAG>] OR PRE('<dMaintenance_End_Date_1_ITAG>') <> [<dMaintenance_End_Date_1_ITAG>]

My problem is however, if someone changes the end date, an email will be sent twice.

the Re evaluate condition wont work since its fulfilling the rule condition requirement.. is there any other work around? does anyone have any better ideas?