When comparing two independent samples, you want to rank all the data together.

Revising your example:

Sample A

 value  rank
    20  7.5
    20  7.5
    20  7.5
    20  7.5
    25  10

and Sample B

value  rank
    1  1
    2  2
    3  3
    4  4
    5  5

What is going on?

Sample B's value of 1 is the lowest ordered value from both samples, so it gets a rank of 1. Similarly for Sample B's values of 2–5. The mean rank for Sample B is therefore $\frac{1+2+3+4+5}{5}=2.5$.

Sample A's values of 20, 20, 20, and 20 occupy the 6th, 7th, 8th, and 9th ranks together, so they each get the average rank of $\frac{6+7+8+9}{4\text{ rank positions}}=7.5$. Finally, Sample A's value of 10 is the largest value from both samples so it gets the highest rank 10. The mean rank for Sample A is therefore $\frac{7.5+7.5+7.5+7.5+10}{5}=8$.

Bonus: To be super explicit: No. The mean ranks of two independent samples of the same $N$ will not necessarily have the same mean ranks.

When comparing two independent samples, you want to rank all the data together.

Revising your example:

Sample A

 value  rank
    20  7.5
    20  7.5
    20  7.5
    20  7.5
    25  10

and Sample B

value  rank
    1  1
    2  2
    3  3
    4  4
    5  5

What is going on?

Sample B's value of 1 is the lowest ordered value from both samples, so it gets a rank of 1. Similarly for Sample B's values of 2–5. The mean rank for Sample B is therefore $\frac{1+2+3+4+5}{5}=2.5$.

Sample A's values of 20, 20, 20, and 20 occupy the 6th, 7th, 8th, and 9th ranks together, so they each get the average rank of $\frac{6+7+8+9}{4\text{ rank positions}}=7.5$. Finally, Sample A's value of 10 is the largest value from both samples so it gets the highest rank 10. The mean rank for Sample A is therefore $\frac{7.5+7.5+7.5+7.5+10}{5}=8$.

Bonus: To be super explicit: No. The mean ranks of two independent samples of the same $\boldsymbol{N}$ will not necessarily have the same mean ranks.

When comparing two independent samples, you want to rank all the data together.

Revising your example:

Sample A

 value  rank
    20  7.5
    20  7.5
    20  7.5
    20  7.5
    25  10

and Sample B

value  rank
    1  1
    2  2
    3  3
    4  4
    5  5

What is going on?

Sample B's value of 1 is the lowest ordered value from both samples, so it gets a rank of 1. Similarly for Sample B's values of 2–5.

Sample A's values of 20, 20, 20, and 20 occupy the 6th, 7th, 8th, and 9th ranks together, so they each get the average rank of $\frac{6+7+8+9}{4\text{ rank positions}}=7.5$. Finally, Sample A's value of 10 is the largest value from both samples so it gets rank 10.

When comparing two independent samples, you want to rank all the data together.

Revising your example:

Sample A

 value  rank
    20  7.5
    20  7.5
    20  7.5
    20  7.5
    25  10

and Sample B

value  rank
    1  1
    2  2
    3  3
    4  4
    5  5

What is going on?

Sample B's value of 1 is the lowest ordered value from both samples, so it gets a rank of 1. Similarly for Sample B's values of 2–5. The mean rank for Sample B is therefore $\frac{1+2+3+4+5}{5}=2.5$.

Sample A's values of 20, 20, 20, and 20 occupy the 6th, 7th, 8th, and 9th ranks together, so they each get the average rank of $\frac{6+7+8+9}{4\text{ rank positions}}=7.5$. Finally, Sample A's value of 10 is the largest value from both samples so it gets the highest rank 10. The mean rank for Sample A is therefore $\frac{7.5+7.5+7.5+7.5+10}{5}=8$.

Bonus: To be super explicit: No. The mean ranks of two independent samples of the same $N$ will not necessarily have the same mean ranks.