bug#15386:

[Daniel Pettersson]

I don't believe this is a bug, but a feature as they say.

This behavior is controlled by `eshell-cmpl-use-paring'

> If t, pare alternatives that have already been used.
> If nil, you will always see the completion set of possible options, no
> matter which of those options have already been used in previous
> command arguments.

After setting the custom to a non-nil.

(setq eshell-cmpl-use-pairing nil)

Will turn on the desired behavior:
    $ ls ~/Dropbox/ ~/Dropb<Tab>
    $ ls ~/Dropbox/ ~/Dropbox/