**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.**