Node [&1]: Node name '&3' for characteristic &2 already exists

What causes this issue?

The nodes NODEID = &V1& have the same node names NODENAME = &V3&. This
is not allowed because neither node is a leaf and both nodes are
assigned to the same characteristic &V2&.
Note that a maximum of 50 characters is available for message variables.
The node name NODENAME = &V3& might not be displayed in its full length.

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?

Try to localize the problem. If the hierarchy is loaded from an SAP
source system, you can check whether the extracted data is correct by
executing transaction RSA3 in the source system. If necessary, check if
the data is modified with a user exit. If the hierarchy is loaded from a
file, check the contents of this file. The problem might also be due to
an error in the transfer rules or in the transformation.
You can identify the two duplicate nodes from the values for the node ID
NODEID = &V1&.
The problem can be caused by identical nodes delivered more than once
from the source. It is also possible that incorrect values for
parameters such as the node name NODENAME or validity period [DATEFROM,
DATETO] are the cause of the problem.
First check if the value for node name NODENAME = &V3& is correct. The
node name consists of the characteristic value for the hierarchy basic
characteristic and the characteristic values of all the characteristics
compounded to this characteristic. Errors creating the node name often
result in duplicate nodes. This problem also occurs if the node names of
all nodes are initial.
If the structure of the hierarchy is defined as time-dependent, the time
validity of a node can be restricted with the fields DATEFROM and
DATETO. Duplicate nodes generally do not occur if the validity intervals
of two nodes do not overlap in time. In this case, check if the fields
DATEFROM and DATETO are correctly filled.
In this case, the duplicate nodes are not leaves. If you want to reuse
an existing subtree at another location in the hierarchy, you can refer
to this subtree with a link node. Possibly one of the two duplicate
nodes was incorrectly not marked as a link node.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.