Group data in a variable

Summary:

I have the following case:

• Within my integration I have a service that returns an array of data with four fields (ID, NAME, PHONE, EMAIL)
• there is a main field (ID) that can be repeated in several lines.
• I need to send only the IDs to another REST service, but without it being duplicated.
• The Rest service only receives line by line, I cannot send it an array of IDs.
• What I require is to group the IDs of the first service in a variable, and then traverse that variable and within the traversal invoke the REST service and send ID one by one.