Can an item of content exist in multiple communities?

Yes, an item of content can exist in multiple communities simultaneously. Community admins can copy an item of content from one community to another using the Bulk content operations feature, in the General section of the Community admin screen. When an item of content is copied to another community, it is not duplicated, it is simply added to that community. This means the same item of content exists in both the original community and the selected target community simultaneously. 

Example:

If an item of content is copied from the Finance community to the Human Resources community, it will appear in both communities at the same time, and will display the same content ID in the URL when viewing it in both communities. 

When an item of content is edited, the changes are reflected in every community in which the item of content exists.