diff --git a/util/check.js b/util/check.js
index 9806bb7..d983b07 100644
--- a/util/check.js
+++ b/util/check.js
@@ -131,6 +131,28 @@ function getAssetInfoFromDB(asset, id) {
   });
 }
 
+function computeHash(filepath) {
+  return new Promise(function(resolve, reject) {
+    let input = fs.createReadStream(filepath),
+      hash = crypto.createHash("sha256");
+    if (!input) {
+      return reject()
+    }
+
+    input.on("readable", function() {
+      const data = input.read();
+      if (data) {
+        hash.update(data);
+      } else {
+        input.close();
+        resolve(hash.digest("hex"));
+        hash = null;
+        input = null;
+      }
+    });
+  });
+}
+
 let photoDB;
 
 require("../server/db/photos").then(function(db) {
@@ -139,6 +161,7 @@ require("../server/db/photos").then(function(db) {
   return Promise.mapSeries(items, function(item) {
     const asset = {};
 
+    console.log("Processing " + item.value);
     return Promise.resolve().then(function() {
       switch (item.type) {
       case "file":
@@ -151,15 +174,17 @@ require("../server/db/photos").then(function(db) {
       }
     }).then(function() {
       if (!asset.stats && !asset.id) {
-        throw "Item " + item.value + " exists neither on disk nor in the DB.";
+        throw "...exists neither on disk nor in the DB.";
       }
     }).then(function() {
       if (asset.id) {
         return;
       }
 
-      console.log("Item " + asset.filepath + " does not exist in the DB.");
+      console.log("...does not exist in the DB.");
+
       asset.size = asset.stats.size;
+      asset.needsHash = true;
       asset.needsUpdate = true;
       return photoDB.sequelize.query("INSERT INTO photos " +
         "(albumId,filename,name,size) VALUES(:albumId,:filename,:name,:size)", {
@@ -169,8 +194,10 @@ require("../server/db/photos").then(function(db) {
       });
     }).then(function() {
       if (asset.stats) {
-        if (asset.modified > asset.stats.mtime) {
+        if (asset.modified < asset.stats.mtime) {
+          asset.modified = asset.stats.mtime;
           asset.needsUpdate = true;
+          asset.needsHash = true;
         }
 
         if (asset.deleted) {
@@ -182,16 +209,79 @@ require("../server/db/photos").then(function(db) {
         return;
       }
 
-      console.log("Item " + asset.filepath + " does not exist on disk.");
+      console.log("...does not exist on disk.");
+      
       if (!asset.deleted) {
         console.log("...setting deleted=1 for " + asset.id);
         return photoDB.sequelize.query("UPDATE photos SET deleted=1 WHERE id=:id", {
           replacements: asset
         });
       }
+    }).then(function() {
+      if (!asset.needsHash) {
+        return;
+      }
+
+      return computeHash(asset.path + asset.filename).then(function(hash) {
+        asset.hash = hash;
+      }).then(function() {
+        return photoDB.sequelize.query("SELECT photos.id,photohashes.*,photos.filename,albums.path FROM photohashes " +
+          "LEFT JOIN photos ON (photos.id=photohashes.photoId) " +
+          "LEFT JOIN albums ON (albums.id=photos.albumId) " +
+          "WHERE hash=:hash OR photoId=:id", {
+          replacements: asset,
+          type: photoDB.sequelize.QueryTypes.SELECT
+        }).then(function(results) {
+          let query;
+  
+          if (results.length == 0) {
+            query = "INSERT INTO photohashes (hash,photoId) VALUES(:hash,:id)";
+            console.log("...creating photohash.");
+          } else if (results[0].hash != asset.hash) {
+            query = "UPDATE photohashes SET hash=:hash WHERE photoId=:id";
+            console.log("...updating photohash.");
+          } else if (results[0].photoId != asset.id) {
+            console.log("...duplicate asset: " + 
+              "'" + asset.album.path + asset.filename + "' is a copy of " +
+              "'" + results[0].path + results[0].filename + "'");
+            asset.duplicate = results[0].id;
+          }
+
+          if (!query) {
+            return asset;
+          }
+  
+          return photoDB.sequelize.query(query, {
+            replacements: asset,
+          });
+        });
+      });
+    }).then(function() {
+      if (asset.deleted) {
+        throw "...deleted. Skipping rest of processing.";
+      }
+      if (asset.duplicate) {
+        throw "...duplicate of " + asset.duplicate + ". Skipping rest of processing.";
+      }
+
+      return exists(asset.path + "thumbs/" + asset.filename).then(function(stats) {
+        if (!stats || stats.mtime < asset.modified) {
+          console.log("...needs thumb");
+          asset.needsThumb = true;
+        }
+      });
+    }).then(function() {
+      return exists(asset.path + "thumbs/scaled/" + asset.filename).then(function(stats) {
+        if (!stats || stats.mtime < asset.modified) {
+          console.log("...needs scaled");
+          asset.needsScaled = true;
+        }
+      });
     }).then(function() {
       if (asset.needsUpdate) {
-        console.log("DB needs to be updated for " + item.value);
+        console.log("...DB needs to be updated for " + item.value);
+      } else {
+        console.log("...does not need to be updated.");
       }
     }).catch(function(error) {
       console.log(error);