Examples:
- They boys have a/the same weight of 40 kg.
- The roads have a/the same distance of 10 km.
We would normally say "the same...", but "a weight/distance of..."
Which is correct?
Neither of them seems natural. We would normally say
All the boys weigh 40kg
But
The boys have the same weight, 40 kg.
would also be possible
All the roads are 10 km long
Roads do not have distance they have length. They may be at a distance from the speaker or from some other feature.