Is 想必一定 a wrong usage? There are many example sentences (763,000). Such as 你这么成熟懂事,想必一定没人疼你.

Intuitively I think it's fluent.

But think carefully, 想必 means "presumably, most probably," is this wrong?
The meaning can be understood without either 想必 or 一定. But I think using 想必一定 conveys a slightly different nuance from that of either 想必 or 一定.

一定没人疼你: very certain presumption

想必没人疼你: still pretty certain but less so than 一定, and also implies that the speaker has evidence to back his/her view. (since 想必 is usually used with (因为)...想必...)

想必一定没人疼你: conveys both of the connotation above. Very certain and implies that the speaker has evidence.

Therefore, I don't think this is a wrong usage. Consider another case of using two presumption words: 大约的确是死了 (from 《孔乙己》). Neither 大约是死了 nor 的确是死了 can convey the subtle connotation of 大约的确是死了, although 大约 and 的确 seems to be contradicting.