Key is already defined; field & cannot be in the key

What causes this issue?

When defining table fields, you added further key fields at the end
after entering a block of key fields followed by a block of non-key
fields.
All the key fields of a table must be in a block at the beginning of
the table.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

All key fields in a table should be entered in a single block.

Error message extract from SAP system. Copyright SAP SE.