In "Mi chamocha ba'elim H', Mi camocha nedar bakodesh" (from the Az Yashir song by the sea, but also between the shma and the amidah in the morning, and elsewhere):
מִֽי-כָמֹ֤כָה בָּֽאֵלִם֙ יְ-ק-וָ֔-ק מִ֥י כָּמֹ֖כָה נֶאְדָּ֣ר בַּקֹּ֑דֶשׁ נוֹרָ֥א תְהִלֹּ֖ת עֹ֥שֵׂה פֶֽלֶא
Why does the second cuf have a dagesh (and thus is pronounced as the harder "C"), while the first has no dagesh (and is thus pronounced as the softer "ch")?
Rabbi Schmuel Mann (of blessed memory) speculated that this is related to how the words were sung, but we had a problem with that because there is nothing special about the trop on the phrase.