72 Pa. Stat. § 5511.16
In case the tax collector or a deputy tax collector shall at any time find, within the taxing district, any resident or inhabitant above the age of eighteen years whose name does not appear upon the duplicate of such taxing district, he shall report the name of such person forthwith to the assessor who made the assessment used by the taxing district.
The assessor shall thereupon promptly certify the said name to the taxing district which made the assessment, which shall then promptly certify such name to the tax collector reporting such name. If the taxing authorities of any taxing district shall at any time find, within the district, any resident or inhabitant above the age of eighteen years whose name does not appear upon the duplicate of the taxing district, it may, by resolution, determine that the name of such person belongs on the tax assessment list and on the tax duplicate and certify the same to the tax collector with direction to collect the proper taxes from such person for the current year and for the preceding year or the two preceding years, if he was liable for such taxes under existing law. The taxing authorities shall at the same time certify such name to the county board for the assessment and revision of taxes or other authority charged with the duty of making the assessment used by the taxing district.
Upon receiving any name as aforesaid, the tax collector shall add the name and assessment of such person to the duplicate of the proper taxing district and proceed to collect the tax or taxes assessed against such person as herein provided.
72 P.S. § 5511.16