Adding column of random floats to data frame, but with equal values for equal data frame entries

I have a column of integers, some are unique and some are the same. I want to add a column of random floats between 0 and 1 per row, but I want all of the floats to be the same per integer.

The code I'm providing shows a column of ints and a second column of random floats, but I need the floats for the same ints, like 1, 1, and 1, or 6 and 6, to all be the same, while still having whatever the float assigned to that int randomly generated. The ints I'm working with, however, are 8 digits, and the data set I am using is about 500,000 lines, so I am trying to be as efficient as possible.

I've created a working solution that iterates through the data frame that has already been created, but creating the random column, then iterating through checking like ints takes long. I wasn't sure if there was a more efficient method.

import numpy as np
import pandas as pd

col1 = [1,1,1,2,3,3,3,4,5,6,6,7]
col2 = np.random.uniform(0,1,12)

data = np.array([col1, col2])

df1 = pd.DataFrame(data=data)
df1 = df1.transpose()