It is not possible to place the same contacts within multiple active campaigns, however you can load the same search in multiple campaigns if only one of them is active.
To help you further understand the scenario, here is an example. Let us say User 1 has created two campaigns, Campaign 1 and Campaign 2. The user also has two search called Search 1 and Search 2.
Keep in mind that Campaign 1 is inactive and Campaign 2 is active.
User 1 has loaded Search 1 in Campaign 1 but now wishes to upload Search 2 in Campaign 2. This is possible, however the software will not give the error message if there are duplicate leads in the list.
The easiest solution to avoid this situation from happening is to follow these steps:
Select the Search tab.
Select Search 1 for Campaign 1 and filter by Search 2. (This way all the people from Campaign 1 that are in Campaign 2 will be shown and it is possible to remove those people manually.)