Observer design error when the desire poles are equals ?

I try to design full order observer with the following

G = [0 20.6; 1 0];

C = [0 1];

obsv_poles = [-10; -10];

ke = place(G',C',obsv_poles);

eig(G-ke'*C);

but i have the following error

Error using place (line 78) The "place" command cannot place poles with multiplicity greater than rank(B).

[Ngoc Thanh Hung Bui]

what are G and C??? did you check the observability of the system yet