<a-scene>
  <a-assets>
    <a-mixin id="red" material="color: red"></a-mixin>
    <a-mixin id="blue" material="color: blue"></a-mixin>
    <a-mixin id="cube" geometry="primitive: box"></a-mixin>
  </a-assets>
  <a-entity mixin="red cube"></a-entity>
  <a-entity mixin="blue cube"></a-entity>
</a-scene>

The entity with red cube will take the properties from the red mixin and the cube mixin in that order. Likewise with the blue cube. Conceptually, the entities above expand to:

<a-entity material="color: red" geometry="primitive: box"></a-entity>
<a-entity material="color: blue" geometry="primitive: box"></a-entity>

Properties of a multi-property component will merge if defined by multiple mixins and/or the entity. For example:

<a-scene>
  <a-assets>
    <a-mixin id="box" geometry="primitive: box"></a-mixin>
    <a-mixin id="tall" geometry="height: 10"></a-mixin>
    <a-mixin id="wide" geometry="width: 10"></a-mixin>
  </a-assets>
  <a-entity mixin="wide tall box" geometry="depth: 2"></a-entity>
</a-scene>

All of the geometry component properties will merge since they are included as mixins and defined on the entity. The entity would then be equivalent to:

<a-entity geometry="primitive: box; height: 10; depth: 2; width: 10"></a-entity>

When an entity includes multiple mixins that define the same component properties, the right-most mixin takes precedence. In the example below, the entity includes both red and blue mixins, and since the blue mixin is included last, the final color of the cube will be blue.

<a-scene>
  <a-assets>
    <a-mixin id="red" material="color: red"></a-mixin>
    <a-mixin id="blue" material="color: blue"></a-mixin>
    <a-mixin id="cube" geometry="primitive: box"></a-mixin>
  </a-assets>

  <a-entity mixin="red blue cube"></a-entity>
</a-scene>

If an entity itself defines a property that is already defined by a mixin, the entity's definition takes precedence. In the example below, the entity includes both red and blue mixins and also defines a green color. Since the entity directly defines its own color, the final color of the cube will be green.

<a-scene>
  <a-assets>
    <a-mixin id="red" material="color: red"></a-mixin>
    <a-mixin id="blue" material="color: blue"></a-mixin>
    <a-mixin id="cube" geometry="primitive: box"></a-mixin>
  </a-assets>

  <a-entity mixin="red blue cube" material="color: green"></a-entity>
</a-scene>