There are many example sentences (763,000). Such as
Intuitively I think it's fluent.
But think carefully, 想必 means "presumably, most probably," is this wrong?
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想必 draws an inference. 你这么成熟懂事 provides the reasoning. 一定没人疼你 is the inference. 想必 can be 'presumbly' in English. You should take 想必 and 一定 separately.
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.
"想必" short for "想来必定" (think it must be) ; "一定"(certainly) seems redundant to me.
"必一定" is like saying "certainly" twice
Either "你这么成熟懂事, 想必没人疼你" or "你这么成熟懂事, 我想一定没人疼你" sounds more nature than "你这么成熟懂事, 想必一定没人疼你" (think it must be, certainly)
And yes, "想必" (think it must be) does mean "presume"; so does "我想一定"
The problem is, if you already said "presumably" then follow it with "certainly" is a logical contradiction
Defination of "想必" :
[presumably; most probably] 表示比较肯定的判断、猜测
I think you are right. "想必一定" is not an accurate use. "想必" is still guessing, while "一定" is making assertion.
Either "你这么成熟懂事,想必没人疼你" or "你这么成熟懂事,一定没人疼你" is more correct.