I am trying to Get multiple contacts from Flow by using a collection of IDs.
In the below image I am looping through each opportunity contact and adding the Contact Id to a collection.
Then I want to get all contacts whose IDs are in the collection.
In APEX this is pretty straight Forward. Is this something we can do in flow.
Once you insert a sObject collection, you should be able to access the record IDs in the collection itself.
To demonstrate, add a screen element and display the sObject collection that you just inserted. You will see a list of opp IDs.
Screenshots below for a sample flow that creates two account sObject variables, adds them to a collection, and inserts the collection. You can see that prior to the insert, if you display the sObject collection, it just shows a commma. After the insert, you see the two IDs.
Flow
sObject collection before insert
sObject collection after insert
My apologies for my comment suggesting you use a set, as you can't specify the use of a set in a Visual Workflow.
You can however use the contains operator in Decision elements to determine whether an item in a collection variable contains the exact same value as a specified variable, or whether an item in an sObject collection variable contains the same exact field values as a specified sObject variable before adding them to a collection.
You'll want to do this sort of testing before adding sObjects from one collection to another collection to avoid this issue. See Flow Design Considerations for more on this.
Best Answer
Flow Get Records cannot perform a single query using a collection of IDs -- you have to loop through the collection and get each record individually. If governor limits are an issue in your use case, you'll have to do this in Apex instead. You can also vote on this Idea.