Importing using the Commerce Data Migration Tool might fail with an error if a relationship definition exists in another catalog. The following message appears in this regard:
ERROR PipelineAbort:The relationship definition '[Relationship Definition entity]' already exists.
To resolve the issue, download and install the hotfix or the patch compatible with the affected product version:
Be aware that the hotfix was built for a specific Sitecore XC version, and must not be installed on other Sitecore XC versions or in combination with other hotfixes. In case any other hotfixes have already been installed on a certain instance, send a request for a compatibility check to Sitecore Support. Note that the ZIP file contents need to be extracted to locate installation instructions and related files inside. The hotfixes must be installed on a CM instance and then synced with other instances using standard development practices.