Migration Multisite D7 to Single D8 I have several D7 sites that I inherited that should have been one site.  We are now trying to migrate to D8 (not all needed modules are available yet on D9).  We would like to migrate them to one site.
When I migrate site 1 nodes and/or webforms/submissions, all goes well.  When I migrate site 2, some of site one items are overwritten because they have the same ids.  How do I configure a migration so it does not overwrite a previous migration?
There is more than one way to migrate Drupal content. Usually if you assign a migration from a database into a different database the assumption is that it's about migrating content into a new structure, without any additional content considerations.
As mentioned in the comment you should export/import the second site content rather than migrate.
You can look into Feeds module to import from a CSV, for example, and you can use Views to create a list of your content then export it into a CSV file. This isn't the only way to go about it, just one suggestion. Try to research the export/import ways of content in Drupal ;)