Selecting multiple regions in a map with multiple fields keeps adding repeated filters

[vitorbaptista]

Say that we're plotting a map with the states in the USA, and I add country to the fields as well. So, when clicking on a state, we'll setup filters for it AND for the country, as in:

``` ?state:USA|state:California

The problem is that when I click on another state, it'll add the country again, as in:

?state:USA|state:California|state:USA|state:Florida

We shouldn't be able to duplicate filters. What we want is:

?state:USA|state:California|state:Florida