Style spec feature_filter should dedupe values for legacy in statements

[dereklieu]

Motivation

A customer reported an issue migrating their styles using Studio in order to use expression syntax. Their legacy filters contained an in statement that referenced duplicate values, for example:

filter: [ 'in', 'parent_0', 'AA', 'AA', 'AB', 'AC', ... ]

This seems to be a valid legacy filter, however upon migration it fails validation for the newer syntax with error output GL JSON Invalid: Branch labels must be unique.

I propose that the convertInOp method to convert legacy filter syntax include a line to remove duplicate string or numerical values. This can only benefit users of this utility by easing migration from a looser validation requirement to a tougher one.

Design Alternatives

Alternatively, we could tighten legacy validations, but that would break lots of working maps. We could also surface a warning in GL when we detect duplicate filter values in legacy syntax, however this could be confusing as the filter would still be valid.