You can Use ‘If -then’ block like this…
Edit: You can use ‘If then’ with ‘Data Changed’ To get Specific data if you need. it’s Preety Simple 
Datachange block is calling whole data from firebase when the app opens even I am not calling any data
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Yes, that’s normal but if you want to show specific data to your App’s user then you can use these blocks i have show you above.
Actualy datachange blocks is not calling data when app is opening that’s ‘Get Data’.
when your Firesbase data got any changes then ‘data change’ will Work.
My problem is different and you solution is different
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I haven’t used any get data block but sill it loads all data
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May i see your Blocks?
When Any data will be get change in firebase then ‘data change’ will get all data from firebase 
One solution, since all data is fetched and suppose you get duplicates in global name list, is when datahcaneged. data changed first set global name to create an empty list and then add items to list
Whenever I open my app the datachange block call entire data even I am not calling anything bor adding any new data
Which data do you want?
I am using firebase
I Mean which data do you want with ‘Data Change’
I tried this solution but problem is still there
Turn On Persist and Your problem will be solve.
To overcome this problem try this
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Datachange block is calling all the data from database without calling any data
Is project bucket correctly defined in designer properties ? Because sometimes when we open project again firebase’s project name changes in designer view
Everything is set properly
Kanishka has given solution for same problem but now it’s now working any more