[fix] DocumentRegistry: only add provider once

Fixes <koreader#5946>.

frontend/document/documentregistry.lua

@@ -121,13 +121,23 @@ function DocumentRegistry:getProviders(file)
 
     --- @todo some implementation based on mime types?
     for _, provider in ipairs(self.providers) do
+        local added = false
         local suffix = string.sub(file, -string.len(provider.extension) - 1)
         if string.lower(suffix) == "."..provider.extension then
+            for i, prov_prev in ipairs(providers) do
+                if prov_prev.provider == provider.provider then
+                    if prov_prev.weight >= provider.weight then
+                        added = true
+                    else
+                        table.remove(providers, i)
+                    end
+                end
+            end
         -- if extension == provider.extension then
             -- stick highest weighted provider at the front
-            if #providers >= 1 and provider.weight > providers[1].weight then
+            if not added and #providers >= 1 and provider.weight > providers[1].weight then
                 table.insert(providers, 1, provider)
-            else
+            elseif not added then
                 table.insert(providers, provider)
             end
         end