How to add values to Firebase Firestore without overwriting?

Question

I have two Activities, I am adding data to Firestore from these two activities individually. But, whenever I add second activity data to Firestore, it is overwriting the fir

Answer 1

I suggest you to add one more document or collection that it will be able to store more just one data values for single user.
You can create a document references for both activities:

firebaseFirestore.collection("Users").document(user_id+"/acitivity1").set(data);
//and  
firebaseFirestore.collection("Users").document(user_id+"/acitivity2").set(data);

Or you can create a sub-collection for it:

firebaseFirestore.collection("Users").document(user_id)
                  .collection("Activities").document("acitivity1").set(data);
//and
firebaseFirestore.collection("Users").document(user_id)
                  .collection("Activities").document("acitivity2").set(data);

More about hierarchical data there.