Is "夛" an alias for "多"? For example, I suppose that "夛田" is one of family names in Japan, is it the same as "多田"?
While dictionaries say 夛 is a variant of 多, there are virtually no occasion where they are interchangeable. In fact, I have just learned 夛 is a variant of 多. (They look so different!)
When you construct a normal sentence, you always have to use 多, which is a very basic kanji all children learn at school. If you used 夛い, people would just think you made some strange mistake.
夛 is a rare kanji which is found only in some family names. 夛 is not a 常用漢字, and it is not even included in 人名用漢字, which means it is illegal to name a new baby using 夛. FWIW, the only person I know who has 夛 in their name is Ken Kutaragi.
In general, you should not change the appearance of kanji used in person names. 久夛良木 is 久夛良木 and not 久多良木. 多田 is not 夛田. For details, see: Use of 旧字体 in Japanese names
夛 is a variant of 多. 音 and 訓's readings are respectively タ and おお・い. So both kanji are same.