CSDL spec (2013-03-12) says:

12.1.1 The Name Attribute

The entity container MUST provide a unique SimpleIdentifier value for the Name attribute.

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It isn't clear whether this uniquess is within the scope of the Schema, or the entire metadata document. For example, consider:

<edmx:Edmx xmlns:edmx="http://docs.oasis-open.org/odata/ns/edmx"
Version="4.0">
<edmx:DataServices DataServiceVersion="4.0" >
<Schema xmlns="http://docs.oasis-open.org/odata/ns/edm" Namespace="ODataDemo1">
<EntityContainer Name="DemoService" IsDefaultEntityContainer="false">
</EntityContainer>
</Schema>
<Schema xmlns="http://docs.oasis-open.org/odata/ns/edm" Namespace="ODataDemo2">
<EntityContainer Name="DemoService" IsDefaultEntityContainer="false">
</EntityContainer>
</Schema>
</edmx:DataServices>
</edmx:Edmx>

Note how there are two DemoService containers, each is unique within the scope of its parent Schema. However DemoService is not unique within the metadata document.