The two words would have been identical in the oldest manuscripts, since they had no accents marks or punctuation, but in any case the two words are both the genitive singular for σπέρμα. They seem to have been accented correctly. This seems to be a case of an obscure rule for accenting nouns within phrases. I ran across this somewhat profane explanation:
Remember the God-given Rule of Greek stress? The one saying that no
stress can go beyond the antepenult, the 3rd syllable from the end?
(If you already forgot it, take another look at it in the blue box at
the top of this page.) Well, combinations like the above make up a
single unit of four syllables, at least the way they are pronounced.
There already is a stress on the antepenult (e.g. μάθετε), so when the
enclitic is adjoined (e.g., -μας), the whole unit makes up four
syllables, the first of which is stressed (e.g., μάθετε-μας). And the
Holy Rule says you drop dead if you do that kind of Greek sacrilege.
So what you do to avoid Zeus’s thunderbolt is to put another stress on
the penult of the entire unit (e.g., μάθετέ μας). Note that this
additional stress is not secondary, i.e., weaker (not as in
“dictionary”), but is produced with the same power as the first
I believe this is the explanation for why σπέρματος has an extra accent in the first clause (τοῦ σπέρματός σου), but not the second (τοῦ σπέρματος αὐτῆς).