Similarly to how you can reference values in previous loop iterations, you are able to index values in repeating sections by using __INDEX. (Or by using the parent index if in a loop already)

This can be used in a formula of an element to provide validation or visibility logic based on previous inputs.

Example 

Say there a series of repeating sections and want to validate that none of the responses for each section are the same.
This would be done via a validation formula on the picklist, within the form. See formula in Figure 1 on the Picklist Element.

Figure 1 - Picklist Validation Formula

This will manifest in the app as in Figure 2. This shows that a duplicate value will trigger the error message defined in Figure 1.

Figure 2 - Picklist Validation in Axsy Mobile