### Version 2.5.17 ### Reproduction link [https://codepen.io/anon/pen/ReNVra?editors=0011](https://codepen.io/anon/pen/ReNVra?editors=0011) ### Steps to reproduce Open the console ### What is expected? ``` "mixinA created" "mixinB created" "ChildComponent created" ``` ### What is actually happening? ``` "mixinA created" "mixinA created" "mixinB created" "ChildComponent created" ``` --- `MixinB.options` still has `mixins`, which gets applied again if used in `mixins: []` or `extends: ` even though `Vue.extend()` has already done that <!-- generated by vue-issues. DO NOT REMOVE -->