Fine, you say you have a contract with the Manufacturer so then what you need to do is the following:
First make sure you the other competitor is selling the same thing and not something similar, that it is the same brand, do a test buy.
Secondly, you need to contact the manufacturer/factory and ask them how this is possible that these products are sold to non-authorized dealers.
Thirdly, you want to go into that rabbit hole of what the distributors are doing and to whom they are selling to, have fun with that because you still can’t touch the reseller if these were bought legitimately. You mentioned that you have to be authorized distributor, what if there is another authorized distributor selling to whomever, you can’t control what the other distributor does.
And another point, if you are a distributor, distribute, sell wholesale don’t compete with the retailer. Then you won’t have to worry about who’s selling with you because you are selling wholesale.