This can happen alot. Last year it happened in our district where 1 elementary and 1 middle school both ran the same fundraiser at the same time! Talk about crazy.
The district PTA Council decided to "step-in" and asked that all schools communicate the timing of their fundraising efforts. They also got to gether with both schools to suggest that the vendor use a little common sense too!
Vendor's often work on commission, and having 2 schools in the same section of town run the same fundraiser really hurts the results for both schools. The vendor saw 2 commissions, and thought they had better coverage.
This vendor was told that if they ever scheduled competing events within the district again, all the schools would refuse to do business with them.
It hasn't happened with them yet this year.