Add option to eliminate duplicates when adding value to "multi" properties.

[Yassine El Yaakabi]

Hi,
I am new to using this plugin.
I've noticed when appending a value to a yaml property of type multi (array of values) it actually keeps duplicates.
It would be so nice if this is not the case and we can configure it to filter duplicate values out.
It will be especially useful when the plugin is used from a script.

[CA Manas Madrecha]

Before appending the value, you could check for arr.includes("value you are appending"). If it is true, don't append, if not, then only append.