Can you have a forehead reduction more than once?

Revision hairline lowering or forehead reduction is technically feasible but not often required. 
If additional lowering is desired it is possible but you won't be able to lower it as much as the first time. It should be noted that it's more difficult to raise the forehead back to it's original position. There are some things you can do to release forehead retaining ligaments to length the forehead and elevate the forehead but this is normally quite minimal. 
Fortunately, if you do your research and find someone who has experience with this procedure it is very rare to need any revisions. Most patients are very happy with the final result.  

Hair transplants are commonly performed after hairline lowering to round off the hairline, but not commonly needed to lower the hairline further than what was achieved with the hairline lowering in the first place.  If someone wasn't satisfied with how low their hairline was after a hairline lowering elsewhere, we do offer them a hair transplant to further lower their hairline.  

If it is just a matter of lowering the hairline to better balance your face you could certainly have a hairline lowering procedure ie a hair transplant. If done correctly, there would be no need to repeat it. However, repeating it is possible if needed. You should include a photo next time to help advise you better.

If hair transplants are used to reduce the forehead it can be done more than once to serially lower the hairline. With a forehead reduction procedure this is very rarely necessary but could be done.

Each time you do a forehead reduction, you are stressing the advancement of the forehead and increase your risk of scarring. A hair transplant lowering works well and does not run a scarring risk. My wife and duaghter-in-law had the hairline lowering procedure with hair transplants and it looks great. This approach does not stress the forehead edge and it does not restrict how much you move the hairline forward.