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SUBMIT YOUR IDEASpoiler:
I was a bit confused by this one. The question states that you want to know "the correlation of merchants visited by customer, for example if a customer visit 7-11 what other stores do they have a high propensity to visit." So my solution looks at all combinations between stores for a customer, while at the same time removing any nulls in the 2nd merchant column which are the result of a customer only visiting one store.