I think you need duplicated for boolean mask and for new column numpy.where:
mask = df_picru['REF_INT'].duplicated(keep=False)
Sample:
df_picru = pd.DataFrame({'REF_INT':[1,2,3,8,8,2]})
mask = df_picru['REF_INT'].duplicated(keep=False)
print (mask)
0 False
1 True
2 False
3 True
4 True
5 True
Name: REF_INT, dtype: bool
df_picru['new'] = np.where(mask, 'duplicates', 'unique')
print (df_picru)
REF_INT new
0 1 unique
1 2 duplicates
2 3 unique
3 8 duplicates
4 8 duplicates
5 2 duplicates
If need check at least one if unique value need any for convert boolean mask - array to scalar True or False:
if mask.any():
print ('at least one unique')
at least one unique