zsh associative array has multiple keys with the same value

Question

I am having trouble with some code and I've discovered that I am getting multiple identical keys in my associative array, which I would have not expected possible. I have tried to get this behaviour in a simple test case and am coming up short - if I assign a second key/value pair to the associative array, the old one is gone (which I would expect).

Here is the code that prints out the key/value pairs:

local key=''
local value=''
for key value in ${(kv)grid}; do
  echo "$key -> $value" >&2
done

and here is some of the output (after I sorted it)

"1000,1000" -> #
"1000,1000" -> #
"1000,1000" -> #
"1000,1000" -> #
"1000,1001" -> #
"1000,1001" -> #
"1000,1001" -> #
"1000,1001" -> #
"1000,1002" -> #
"1000,1002" -> #
"1000,1002" -> #
"1000,1002" -> #
"1000,1003" -> #

You can see there are multiple key/value pairs with the same key (and in some cases the same value too).

This occurs on both z/OS Zsh 5.8.1 and Mac Zsh 5.9

I expect I have a bug in my code, but if someone knows how I can be getting these multiple key/value pairs into the associative array, I would be grateful.

Edit: adding more information as requested:

Here is how I declare the grid:

declare -A grid

Here is how I set a 'cell' in the grid:

set_cell()
{
  local srow=$1
  local scol=$2
  local sc=$3
  local skey="${srow},${scol}"

  grid["${skey}"]="$sc"}
}

Here is how I 'get' the value of a cell:

get_cell()
{
  local grow=$1
  local gcol=$2
  local gkey="${grow},${gcol}"
  local gval=${grid[(k)"${gkey}"]}
  echo "${gval}"
  return 0
}

I call the get and set routines as:

  local c
  c=$(get_cell ${row} ${lcol})

and:

local out='O'
set_cell ${row} ${rcol} "${out}"

where the row and col variables are integral.

I also added:

echo "Type of grid: ${(t)grid}" >&2

just before the loop dumping out the values and I get:

Type of grid: association