There's only one zimbraPrefMailForwardingAddress (exposed in preferences), however there can be multiple zimbraMailForwardingAddress (the user is not shown their existence but they can be set in the admin console - do note that any bounces to them would reveal the forward though).
zmprov ma firstname.lastname@example.org zimbraPrefMailForwardingAddress email@example.com
zmprov ma firstname.lastname@example.org zimbraMailForwardingAddress email@example.com
zmprov ma firstname.lastname@example.org +zimbraMailForwardingAddress email@example.com
zmprov ma firstname.lastname@example.org +zimbraMailForwardingAddress email@example.com
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