Grouping by multiple columns to find duplicate rows pandas

You need duplicated with parameter subset for specify columns for check with keep=False for all duplicates for mask and filter by boolean indexing:

df = df[df.duplicated(subset=['val1','val2'], keep=False)]
print (df)
   id  val1  val2
0   1   1.1   2.2
1   1   1.1   2.2
3   3   8.8   6.2
4   4   1.1   2.2
5   5   8.8   6.2

Detail:

print (df.duplicated(subset=['val1','val2'], keep=False))
0     True
1     True
2    False
3     True
4     True
5     True
dtype: bool