How to replace each entry for id with max() for that id?

Question

I have data frame like -

id    col1
1      5
1      3
1      6
1      8
2      1
2      2

I want to replace all values for an id with max for that id. output should look like -

id    col1
1      8
1      8
1      8
1      8
2      2
2      2

I am not sure how to approach this. How can i do this?