Finding and Copying Conditional Duplicates

Hi,

I have a sheet with thousands of rows of data, with about 20 columns or so each. Does anyone know what functions I should/can use to search the sheet for duplicates across columns A and B? Essentially, I want to find all rows whose inputs for columns A and B have a match(es) somewhere in the sheet, then copy them all into a new sheet. To clarify, I want to copy over the whole rows into the new sheet.

Apologies if this is confusing. A quick example: rows 3, 450, and 1,200 all have the identical values for both A and B, and thus will be listed in the new sheet. 

Thanks for any and all help/advice you can provide!