refresh of this patch for octave-6.x.x:

diff -r 62c1443babd8 libinterp/octave-value/cdef-class.cc
--- a/libinterp/octave-value/cdef-class.cc      Sun Mar 10 17:02:27 2019 -0700
+++ b/libinterp/octave-value/cdef-class.cc      Mon Mar 11 19:57:31 2019 +0100
@@ -1062,6 +1062,8 @@
                 for (auto& prop_p : *pb_p->element_list ())
                   {
                     std::string prop_name = prop_p->ident ()->name ();
+                    if (retval.find_property(prop_name).ok())
+                       error ("property %s already defined", prop_name.c_str());

                     cdef_property prop = cdm.make_property (retval, prop_name);

octave happily parses this class:

classdef zz2
    properties
        p1
        p1
    end
    properties (Dependent)
        p1
    end
    methods
        function val = f(obj)
            val = 'f';
        end
    end
end

A side effect of the multiply defined properties is that method f can not be found: the next script

zz2_i = zz2;
f = zz2_i.f

gives this error message:

+verbatim+
error: subsref: unknown method or property: f
error: called from
    run_zz2 at line 2 column 3

Also octave allows the same property name in a class and a superclass:

classdef zz < zzs
    properties
        p1 = 2;
    end
end

classdef zzs
    properties
        p1 = 1;
    end
end

which results on execution of this script

zz_i = zz;
p1 = zz_i.p1

in

p1 =  2

Is the case for both stable and dev.
I didn't find this reported in an earlier bug report, but can hardly imagine this has gone unnoticed to date.