Can multiple companies use the same DBA name?

Essentially, you are claiming is that the tenant has stolen your goodwill, an intangible asset.  Also, you appear to be claiming that, by virtue of the use of the restaurant trade name over many years, it obtained the status as a common law trademark, which the new tenant is infringing.  I assume that you never applied for a formal trademark registration; while such a registration would provide you with additional benefits above and beyond the benefits of a common law trademark, including additional damages which can be recovered for infringement, you can sue for infringement of a common law trademark.  The tenant may have valid defenses to such a claim, including defenses based on your "not being able to continue the fight" earlier, so I can't predict whether your suit would succeed.  Also, you would probably have to prove that the mark had obtained "secondary meaning" among consumers, meaning that consumers had come to associate the mark with your restaurant, and would be confused by the tenant's use of the mark into believing that they were dealing with your restaurant.  Consumer surveys are often required to meet the burden of proof on such a claim, and those surveys can be very expensive.