if someone has the same bmi does that mean they are the same skinniness?

Nope.

Somebody with more muscle mass will appear thinner than someone with less muscle mass who weighs the same number of pounds and is of the same height (because muscle is denser [has a higher ratio of weight to volume] than fat tissue).

To give a concrete example: say you have two men who are both 6' tall and both weigh 200 pounds, but one is an athlete and the other is a couch potato. They will both have the same BMI (since they both weigh the same and are of the same height), but one will look better and be in better shape than the other.

This is widely considered one of the primary drawbacks of using BMI to gauge fitness/health.

What is skinniness?

If you mean measurement of different body parts like waist, hips, chest, etc, the answer is no. BMI takes into consideration only the height and the mass. The body is composed of fat, muscle, skeletal tissue, body fluids, etc. Every person differs in the percentage of these different types in their body. Two people with the same BMI can have different fat % and so they would have totally different measurements.

Now even fat % is not the perfectly accurate indicator of measurements. Fat could be stored under the skin which will add to the measurements or be stored around the internal organs and not reflect so much in your body measurements

The BMI index is calculated using weight in the numerator and height in the denominator. Weight isn't a real good measure of skinniness, since muscle has a higher density than fat. You could have two people with exactly the same height and weight, but one of them has heavy bones and is full of muscle while the other one has light bones and has a lot of body fat.

Weight and BMI should be used as a measure of how you are progressing on your diet. Do not use it to compare yourself with others, because there are a lot of factors in weight (like muscle, fat, bones and water)

BMI is also used to group people in categories like underweight (<18.5), normal weight (18.5-24.9), overweight (25-19.9) and obesity (30 or greater).

No- it sould depend on their height, weight, and fat distribution. Person 1 could be a pear shape, person 2 could be an apple shape, so they could look totally different due to the distribution of fat on their body.

not at all according to the BMI Im fine according to a more accurate fat % test, Im obese