Letter 673C is usually used for some general communication of various IRS procedures - often an explanation of return handling after duplicates returns are filed by you.
The letter often states that the IRS is combining the duplicate returns. In fact, they don't double the amounts. It is a reconciliation of the two returns. However, they can be wrong so it is extremely important to verify that what they've done is correct.
Response to Letter 673C
It is best to gather your file & information from the IRS to compare with your records. You may have to file an Amended Return (e.g., Form 1040x) or involve the Taxpayer Advocate. You simply want to make sure the correct figures from the return are reflected in your Account Transcripts.
If you agree with the IRS, then no action is needed. But, if you disagree or are unsure, please review the Prep Steps and read our Blog topic on ID Theft. Of course, you may always contact Mr. Hopkins directly!