I replied to your other forum post on the same question, but will reply again here for other users who may have similar questions.
By default, IPUMS USA data are person-level microdata, meaning each row or observation is a person. Each column is a variable that describes a person-level characteristic or household-level characteristic; household-level variables are automatically appended to person records.
In IPUMS USA, the two variables SAMPLE and SERIAL uniquely identify households. The three variables SAMPLE, SERIAL, and PERNUM uniquely identify persons. While SAMPLE and SERIAL uniquely identify households, if your data extract includes person records as well as household records (as you stated), you will still have multiple observations associated with many of the households; many households include multiple household members. If your data extract includes only household records, then you will have just one observation per household.