Error, on datepicker dialog open

[walkerz88]

Hi! When i open md-datepicker, or select date, i've got an error in console:
[Vue warn]: Duplicate keys detected: '1'. This may cause an update error.

code:
<md-datepicker v-model="event.date_end"/>

event: {
date_end: null
}

But datepicker still works fine, just show this error

Please see screenshot: http://prntscr.com/i0bpv3

[Samuell1]

Duplicate of #1376