Does data cleaning automatically happen monthly?

hi kowas2 

Since you steps involve combining worksheets then removing the duplicates, Power BI will remove the duplicates when you refresh the semantic model. Just be aware though that that Power BI will keep the first row it sees for each distinct value which may not be necessarily ordered by month.

Hello kowas2,

In Microsoft Power BI, your data cleaning steps (like Remove Duplicates) are saved in Power Query and only run when you refresh.

• Add new file → nothing happens yet
• Click Refresh (or use scheduled refresh)
• Power BI recombines files and removes duplicates again

You don’t need to remove duplicates manually each time.

If you want it monthly without action:

• Set up Scheduled Refresh in the Service

Docs:
Working with duplicate values 
Data refresh in Power BI