I've read that ידים is to be pronounced ya- DAI - im, yet שמים is to be pronounced sha-MAI-yim.
Both end the same, but are pronounced differently. (This was from a text purporting to teach "Biblical Hebrew, meaning as it was pronounced in ancient times).
Is there a rule I am missing? Seems strange, as the orthography is exact for the pair.
EDIT: I would like to thank Dave for pointing out later on in the book the author completely contradicts her original phonetic rendering and spells all of the duals with the YIM ending.
Also, I initially spelling yadaim with two yud and I apologize for the error.
EVEN MORE INTERESTING: It appears in Modern Hebrew there has been a spelling change to include two yud whereas Biblical only used one, seemingly to prevent this confusion.