Specify region- or group- dependant coefficients of friction

Hi Everybody

I wanted to set the friction to different values, depending on where the particle is in the model, e.g. in a hopper, and in the process below the hopper.

My approach was to:
- define a region, REG_hopper
- associate a group of particles with this region
- set the friction first for "all", and then for the group using "fix property/global"

The steps below shows only the applicable commands:
region REG_hopper block x_lo x_hi ... z_lo z_hi units box
...
group GRP_hopper region REG_hopper
...
fix m1 all property/global coefficientFriction peratomtypepair 1 ${mu_1}
fix m2 GRP_hopper property/global coefficientFriction peratomtypepair 1 ${mu_2}

When I follow the above, I get:
ERROR: Error in fix property/global. There is already a fix that registers a variable of the same name

Is there a better way to do this?

Thank you

Alexander