Vue.js Multiple definitions of a property not allowed in strict mode
You can't use v-model and :checked at the same time. When you added v-model="selectedGender", you provided it a way to determine the checked status based on the value of selectedGender. That caused it to create this code:
checked: t._q(t.selectedGender, "MALE")
When you also added :check="selectedGender === 'FEMALE'" you caused it to add this other way to set the checked status:
checked: "FEMALE" === t.selectedGender,
You can't have both. Just remove the :checked= to fix this.
To build on @Charles answer, this is true of other duplicate properties as well. For example you can't have :value="myProperty" and value="true" declared as it will create similar code mentioned in Charles answer.
Simply removing the duplicate properties fixes these kinds of issues.