Make plugin reject duplicate properties

[Astrael1]

User may provide above properties. Especially after implementing #78 In this case they should be informed, that the plugin doesn't work like that.
https://github.com/hudochenkov/stylelint-order/pull/196/files/e57485483cff773842bc5a18a974044570ef7272..9639ef33c0002340cb352dbf29a9e94ad597ce86#diff-05911a0ffa1a3141834350d8bac82c2cd58507c126657ab749570f0f88d1dfc6

How about implementing that @hudochenkov?

[hudochenkov]

I don't think it is necessary. For the whole time plugin exists no one ever mentioned that this is a problem. So I see no reason fixing what is not broken :)