They are not the same meaning Once every week emphasize one time on a weekly basis

The others make a good point, but there is a small detail in the first sentence that implies that the two sentences can be used interchangeably. 1. Please provide "a" sales report on a weekly basis (the word "a" suggests one report) 2. Please provide a sales report "once" every week. (the word "once" means one time) One sales report weekly = One sales report every week.

The first means you can provide it each week, but it doesn't mention how many you will provide. The second means one per week, so there is just one sending of the sales report