Enable declaration-block-no-duplicate-properties style lint rule; Fix existing case.

[jorgefilipecosta]

This PR enables rule declaration-block-no-duplicate-properties.
And removes a case where we had a duplicate property.
I don't see a reason for disabling this rule as having a duplicate property in the same block is something we should avoid (just sents useless bytes over the wire).

[talldan]

Agreed, especially if this is the only instance as it looks like a dev mistake.

The only reason I can think for using duplicate properties are browser compatibility fixes (like the old school way of defining background-image: {browser-prefix}-linear-gradient( // ... ). I guess these are all handled by Post CSS these days?

[jorgefilipecosta]

The only reason I can think for using duplicate properties are browser compatibility fixes (like the old school way of defining background-image: {browser-prefix}-linear-gradient( // ... ). I guess these are all handled by Post CSS these days?

Yes, I think post CSS handles these, cases but if there is a need to use a duplicate rule I guess we can use an exception and ignore the rule in that specific case.