Rerunning a macro error and recommendation for a bug-free ROOT version

[Wile_E_Coyote]

How to run a program twice (or more)

Macro unloading / reloading does not work in (is not supported by) ROOT 6 (as of 2019.07.02).

Note: you should be able to use:

root [0] .x histogram1.C
root [1] histogram1();