SQL Full Outer Join W/ Coalesce

Why does this query produce duplicates in some scenarios?

Table_1   
ID   
1  
2  
3  

Table_2  
ID  
1  
2  
4  

Table_3  
ID  
1  
3  
4  

Query:

SELECT COALESCE(Table_1.ID, Table_2.ID, Table_3.ID)  
FROM Table_1

FULL OUTER JOIN TABLE_2  
ON Table1.ID=Table_2.ID  

FULL OUTER JOIN TABLE_3  
ON Table1.ID=Table3.ID;

Result:

1  
2  
3  
4  
4  

The query duplicates all values where T1 is null and T2/T3 share the same value. Duplicates are removed for any other combination.