2 files changed, 56 insertions, 25 deletions
diff --git a/src/MdBlocks/CLI.hs b/src/MdBlocks/CLI.hs
index 5c2bc29..3849220 100644
--- a/src/MdBlocks/CLI.hs
+++ b/src/MdBlocks/CLI.hs
@@ -81,10 +81,10 @@ run (Extract rawAddress) =
           blocks <- loadBlocks (addressFile address)
 
           case resolveBlock address blocks of
-            Nothing ->
-              die ("mdblocks: block not found: " ++ rawAddress)
+            Left err ->
+              die ("mdblocks: " ++ formatResolveError address err)
 
-            Just block ->
+            Right block ->
               T.putStr (blockContent block)
 
 run (ShowMeta addr) =
diff --git a/src/MdBlocks/Resolver.hs b/src/MdBlocks/Resolver.hs
index 29c1658..1713a16 100644
--- a/src/MdBlocks/Resolver.hs
+++ b/src/MdBlocks/Resolver.hs
@@ -1,37 +1,68 @@
 module MdBlocks.Resolver
-  ( resolveBlock
+  ( ResolveError(..)
+  , resolveBlock
+  , formatResolveError
   )
 where
 
-import Data.Text (Text)
+import Data.List (intercalate)
+import qualified Data.Text as T
 
 import MdBlocks.Address
 import MdBlocks.Types
 
+data ResolveError
+  = BlockNotFound
+  | DuplicateBlockId [Block]
+  deriving (Show, Eq)
+
 resolveBlock
   :: Address
   -> [Block]
-  -> Maybe Block
-resolveBlock address =
-  findByFileAndId
-    (addressFile address)
-    (addressId address)
+  -> Either ResolveError Block
+resolveBlock address blocks =
+  case matches of
+    [] ->
+      Left BlockNotFound
 
-findByFileAndId
-  :: FilePath
-  -> Text
-  -> [Block]
-  -> Maybe Block
-findByFileAndId fp ident =
-  go
+    [block] ->
+      Right block
+
+    many ->
+      Left (DuplicateBlockId many)
   where
-    go [] =
-      Nothing
+    matches =
+      [ b
+      | b <- blocks
+      , blockFile b == addressFile address
+      , mdId (blockMeta b) == Just (addressId address)
+      ]
+
+formatResolveError
+  :: Address
+  -> ResolveError
+  -> String
+formatResolveError address err =
+  case err of
+    BlockNotFound ->
+      "block not found: "
+        ++ showAddress address
+
+    DuplicateBlockId blocks ->
+      "duplicate block id '"
+        ++ T.unpack (addressId address)
+        ++ "' in "
+        ++ addressFile address
+        ++ "\n  found at "
+        ++ formatLines blocks
 
-    go (b:bs)
-      | blockFile b == fp
-      , mdId (blockMeta b) == Just ident =
-          Just b
+formatLines :: [Block] -> String
+formatLines blocks =
+  "lines "
+    ++ intercalate ", " (map (show . blockStart) blocks)
 
-      | otherwise =
-          go bs
+showAddress :: Address -> String
+showAddress address =
+  addressFile address
+    ++ "#"
+    ++ T.unpack (addressId address)