Duplicate values on time span

I think this is what you need:

Find the names of the customers that have 2 different visit dates not more than 14 days apart.

Let's say your table that stores visits is V, you've got CustomerID and VisitDate in there. Then you also have a dimension table with your customers C where you store unique CustomerID's. C joins to T in a 1:many fashion on CustomerID. Then you could add a column [2 Visits Within 14 Days] to C:

 [2 Visits Within 14 Days] = -- calculated column without the use of context transition
var __custId = C[CustomerID]
var __visitDates =
    SUMMARIZE(
        FILTER (
            V,
            V[CustomerId] = __custId
        ),
        V[VisitDate]
    )
var __2VisitsExist =
    NOT ISEMPTY(
        FILTER(
            CROSSJOIN(
                SELECTCOLUMNS (
                    __visitDates,
                    "FirstVD", V[VisitDate]
                ),
                SELECTCOLUMNS(
                    __visitDates,
                    "SecondVD", V[VisitDate]
                )
            ),
            [SecondVD] - [FirstVD] < 14
            && [SecondVD] > [FirstVD]
        )
    )
return
    __2VisitsExist

Best

Darek