from collections import Counter

owner_id = ['owner1', 'owner2', 'owner3', 'owner2']
media_id = ['media1', 'media2', 'media3', 'media4']

owner_stats = Counter(owner_id)

out = {}
for index, owner in enumerate(owner_id):
    if owner_stats[owner] == 1:
        out[owner] = media_id[index]

print (out)