Verse 38 Tithe of the tithes - The tithes of all the produce of the fields were brought to the Levites; out of these a tenth part was given to the priests. This is what is called the tithe of the tithes. The law for this is found, Num 18:26.

Neh 10:38

The priest shall be with the Levites when the Levites take tithe. Some representative (or representatives) of the priestly order was to be present whenever the Levites received their tithes, to take note of the quantity, and prevent the Levites from depriving the priests of their due share—the tithe of the tithe. This tenth, being thus ascertained, was to be conveyed to Jerusalem at the expense of the Levites, and deposited in its appropriate store-chamber.