For each language, only a single word entry is permitted. However, each word can have multiple specializations. When a word is first registered a default specialization is always created, which will match what is passed in. Subsequent calls with different specializations will only update the given specialization. The exact repacement that will be used is determined by the following order of preference:
voice name > language dialect > provider name > default
For example, a replacement specified for voice name sara will be picked over a replacement specified for provider azure.