What is the word used to describe the redundancy of using two synonyms to define one thing?

• Incorrect: a baby lamb
• Correct?: 'a baby sheep', or just 'a lamb'

Is there even one word used to describe this duplication? Or would you just identify the mistake as "a redundant word"?

It seems that the linguistic term is "redundancy":

In English usage, redundancy is usually defined as the use of two or more words that say the same thing, but we also use the term to refer to any expression in which a modifier’s meaning is contained in the word it modifies (e.g., early beginnings, merge together—many more are listed below). Think of redundancies as word overflows.

This list is far from complete, and we’re developing it organically (i.e., adding redundancies as they come up in our work) rather than compiling the list by stealing from other online sources, which would be too easy. If you feel strongly that any redundancy should be added here, please comment.

Or, as Josh61 noted, pleonasms, specifically semantic pleonasms:

Pleonasm (/ˈpliːənæzəm/, from Greek πλεονασμός pleonasmos from πλέον pleon "more, too much") is the use of more words or parts of words than is necessary for clear expression: examples are black darkness, or burning fire. Such redundancy is, by traditional rhetorical criteria, a manifestation of tautology.

Semantic pleonasm is a question more of style and usage than of grammar. Linguists usually call this redundancy to avoid confusion with syntactic pleonasm, a more important phenomenon for theoretical linguistics.