How to use union operator

Consider the below code where I'm generating two random Population A and P. I need to make union of these two population. I tried using union operator, But it's not working.

clc; clear all;

varMin   = -5;           
varMax   =  5; 
D        =  3;    % Dimension
VarSize  = [1 D]; % Decision Variables Matrix Size
NP1      = 10;    % Population 1 size
NP2      = 3;     % Population 2 size
empty_individual.Position   = [];
empty_individual.Cost       = [];

P  = repmat(empty_individual, NP1, 1); % Population 1
A  = repmat(empty_individual, NP2, 1); % Population 2

CostFunction = @(x) sphere(x);

%%Generating random population 1

for i = 1:NP1
    P(i).Position = unifrnd(varMin, varMax, VarSize);
    P(i).Cost     = CostFunction(P(i).Position);
end

%%Generating random population 2
for i = 1:NP2
    A(i).Position = unifrnd(varMin, varMax, VarSize);
    A(i).Cost     = CostFunction(A(i).Position);
end

sphere.m

function ret = sphere(x)
    ret = sum(x.^2);
end

How to make union of A and P.

Answer 1

[KSSV]

% megre two structure 
iwant = [P ; A] ;
% get unique values 
[val,idx] = unique([iwant.Cost]) ;
iwant = iwant(idx) ;