mybatis-3: Space in typeAlias results in the IllegalArgumentException: "Result Maps collection already contains value for"

If you have a trailing space in the alias attribute of a typeAlias definition, like this:

<typeAliases>
  <typeAlias alias="User "  type="com.example.domain.User" />
</typeAliases>

at runtime you get an exception like this:

java.lang.IllegalArgumentException: Result Maps collection already contains value for com.example.persistence.UserMapper.mapper_resultMap[xxx]_association[xxx]

where the map and association referenced in the exception are ones defined in your mapper.

Removing the trailing space fixes the problem, but it took a long time to track down.