If you prepared the K-1 for a partnership or any other pass-through entity and you want to amend the return issuing the K-1, then of course you should amend the return and mail separately.
If you omitted the K-1 and you are now amending the return to add the K-1, then you can include the copy of the K-1 along with the explanation of why you are amending your return. Then mail your amended return.
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