QUESTION:
Why do I get an error message when mapping from the S7 project into an OS? I have a data block each in two different ASs. The data blocks have the same symbolic name but different structures.

ANSWER:
The same structure variable is created for both blocks from the different ASs. However, since the structures are different, this leads to an error message when mapping. This behavior is a system property.
Reason:
In WinCC you must assign unique names that are derived from the engineering. In the case of block types, it is presumed that block types with the same name are identical in code, interface and attributes, because the block type name is used for naming the associated WinCC structure type. Because the data of several S7 programs is operated and monitored by one OS, it is presumed that the associated block types in the assigned programs are identical.
Global DBs are mapped in WinCC on a WinCC structure type with the name of the global DB (either DB<Number> or symbolic name) and to exactly one instance (e.g. S7 program (1)_DB1) of it.
It is quite usual to give different symbolic names in a project to data blocks with different contents.