Trademark Law is subject to the principle of independence. This means that each form mark is protected within the limits of its class, its registration or the territory where protection is sought. As a result, the protection afforded to a trademark owner is limited to the class of goods covered by the registration. For example, if a company registers a trademark to commercialize shoes, that same mark could be registered by another to sell furniture.
It is usual to find several identical "word trademarks" registered for different classes of goods or services. If you want to register a mark already registered, the USPTO will compare the class of goods for which the trademark has already been appropriated with the class you have indicated in your application. If the goods are not substantially similar or related and there is no likelihood of confusion your registration mat be accepted.
When it comes to studying the availability of a trademark, it is also important to consider the geographic market covered by the current trademark owner. If the company that registered the trademark sells its goods nationally, it may be difficult to register again that same trademark. However, if the company does business locally, the odds of getting a registration are higher. It is also worth mentioning that there are some marks that are protected in an exceptional manner and beyond the limits of the class that covers its registration. This is the case of famous trademarks, which have acquired such a level of distinctiveness in the market, that it is unlikely that further registrations of the same mark will be allowed.
To know whether a trademark is well-known the USPTO considers among others, the mark's level of commercial recognition, the marketing channels used by the trademark owner, the degree of care likely to be exercised by purchasers in selecting goods and/or services, the existence of foreign registrations, etc. Given the level of recognition of a famous trademark, under the Lanham Act, the owner of a well-known trademark may object to the registration of another mark that is identical or substantially similar. The owner may also bring an action in U.S. federal court for trademark infringement.
Finally, it also important to consider that the mark you intend to register may also be of the interest of others, maybe because the trademark has become popular and it is a word commonly used in the mark or may because it's actually losing distinctiveness. If you intend to register a mark already used or appropriated by another, it is important to consider whether the mark has enough distinctiveness and its recognition is not being diluted by its use in commerce. Trademark dilution occurs when someone uses a famous mark in a manner that diminishes its capacity to identify and distinguish goods or services. If the mark is subject to a risk of dilution, it may better to abandon it.
In general terms, it is possible to register a mark already registered if the goods or services marked by the current trademark owner are different from those covered by the new registration. However, it good not to lose sight of the fact that these analyses must be made always on a case-by-case basis.