Eccoci.

The number $7$ is special, because $7$ is $111$ written in base $2$, and $11$ written in base $6$ (i.e. $7_{10} = 11_6 = 111_2$). In other words, $7$ is a repunit in at least two bases $b \gt 1$.

We shall call a positive integer with this property a strong repunit. It can be verified that there are $8$ strong repunits below $50$: $\{1,7,13,15,21,31,40,43\}$.
Furthermore, the sum of all strong repunits below $1000$ equals $15864$.