Some commercial leases provide for exclusivity within a type of service does yours? If not, then your landlord doesn't owe you any contractual or other duty to prevent competition within the site and isn't restricted in who they choose to rent to.
So your fight, if you have one regarding trademark infringement due to actual consumer confusion, is with the other tenant. It does sound like you'd have a trademark infringement claim, which hopefully your lawyer is pursuing.
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This seems like you need to bring your lease to an attorney for review. You should also ask about trade name protection.
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I agree with both attorneys who previously answered your request. Based on what you've written, it does not appear that you have any grounds for pursuing a claim against your landlord. It is common, and frankly necessary, to negotiate an exclusivity arrangement with one's landlord prior to signing a lease. If your lease does not have such a provision, there may be some other avenues for pursuing the landlord depending on the other provisions in the document (or lack thereof.) However, without reviewing a signed copy of your lease, I cannot make a firm answer.
It also appears that you may have a number of claims against your neighboring tenant including a potential trademark violation. You should be certain to make an appointment with an attorney in your area who has significant experience in small business litigation to determine the best course of action.
Please do feel free to make contact with me if you have additional questions.
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