Well, for me it's one of the similarities that people see very clearly as they're suggested that they should see that - and completely within a range of siblings likely learning and writing together while growing up.
I did this insanity:
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Capital "I" - no resemblance.
"Know" and "m" - kinda similar, but
she doesn't really do much of ' while they're pretty huge in the note.
Small "t" with a loop in the note, no loops in DA's handwriting.
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"g" in the note has angle, no angle in DA's "g".
Completely different capital "H". Pretty similar looking "Houston".
"p" & capital "L" also similar.
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Similarities...
then completely different "b".
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All her "i" seem to be straight lines if it's first letter of the word, in the note they're similar to her "i" in the middle of the word.
One capital "S" similar, other not so much, and she seems to have a lot of confidence while writing it, in the note it looks kinda hesitant, like the author wasn't very experienced in writing.
Could go either way with "w" and "W".
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Some similarities again.
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But then there is this strange as hell "T" in the letter, completely different from hers.
None of her "c" has this tiny triangle shaped something that most of "c" in the note had. Completely different "c".
Numbers from the envelope vs. DA's numbers:
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That straight line "1" holds striking resemblance but more complex numbers are pretty different.