How do I find all the values that repeat every year?

An alternate suggestion.  Are you simply wanting to count the number of period a customer does business with you?  For example, if a customer does business with you through 2016 and 2017, you want to get a count of 2?  The below method will let you specify that period, using a column from a date table.

Start by creating a simple distinct count measure

Customer Count = DISTINCTCOUNT(Table[Customer Name])

Next we iterate that count across whatever time period you want to do:

Customer Repeats = SUMX(
	VALUES(DateTable[Year]),
	[Customer Count]
)

Now put [Customer Repeats] into a table with 1 Customer Name as your row labels.