Periodic schedule deduplication does not work when there are same elements in more than 2 count

[lfathimakn]

When creating a schedule from a periodic schedule, there is a deduplication logic if we pass "combinePeriodsIfNecessary" flag as true:
https://github.com/OpenGamma/Strata/blob/main/modules/basics/src/main/java/com/opengamma/strata/basics/schedule/PeriodicSchedule.java#L453-L462

But if there are more than 2 duplicates of same element, the deduplication is only removing first instance of duplicate.
Eg:
adj = ["2020-01-01","2020-01-02","2020-01-03","2020-01-06","2020-01-06","2020-01-06","2020-01-07"]
After deuplcation:
adj = ["2020-01-01","2020-01-02","2020-01-03","2020-01-06","2020-01-06","2020-01-07"]