def single(list) do
   Enum.uniq_by(list, &(%{"source" => &1.source}))
  end

Here is my one line ugly try

iex> list |> Enum.reduce(%{}, fn %{"source" => s} = x, acc -> Map.put_new(acc, s, x) end) |> Enum.map(fn {_k, v} -> v end)        
[
  %{"distance" => "4", "source" => "source_1"},
  %{"distance" => "5", "source" => "source_2"}
]

I use Map.put_new to ensure only first source is persisted

For note, your sources ++ p["source"] expression is evaluated here but the result is thrown away since it’s not being returned.

And it looks like you aren’t just wanting a unique one based on the name, but also the one with the highest value in each set, thus I’d probably do either this:

Enum.group_by(list, & &1["source"]) |> Enum.map(fn {_, v} -> Enum.max_by(v, & &1["distance"]) end)

Or this:

Enum.sort_by(list, & {&1["source"], &1["distance"]}, &>/2) |> Enum.dedup_by(& &1["source"])

Or any of a variety of other ways.