Some prophets were king-makers, some were king-breakers. The latter is what's going on here. This anonymous prophet is bringing a word of judgment against the godless king, Jeroboam. Jeroboam decided on his own to eschew the Temple of the Lord in Jerusalem and build his own temple and altar at Bethel.

The prophet cried out against the altar in prophetic agony and curse. The repetition of the word "altar" emphasizes the reason for the denouncement. 300 years after this prophecy was spoken, a king named Josiah reigned in Judah (2 Ki. 23.15-16), destroyed this altar and intentionally defiled it by burning human remains on it so it would never be used again.

What does this verse mean?

By the word of the Lord he cried out against the altar: “Altar, altar! This is what the Lord says: ‘A son named Josiah will be born to the house of David. On you he will sacrifice the priests of the high places who make offerings here, and human bones will be burned on you.’”