diff --git a/UPGRADING.md b/UPGRADING.md
index 7c846ca..106fe37 100644
--- a/UPGRADING.md
+++ b/UPGRADING.md
@@ -1,3 +1,11 @@
+# (2023-02-12) RevBank 3.8
+
+## Update your `revbank.plugins`
+
+Deduplication is moved from individual plugins to a plugin that does that. If
+you want to keep deduplication of cart items, and you probably do want that,
+add `deduplicate` to `revbank.plugins` just below `repeat`.
+
 # (2022-12-25) RevBank 3.6
 
 ## Update your `revbank.plugins`
diff --git a/lib/RevBank/Cart.pm b/lib/RevBank/Cart.pm
index ba6e77f..7c7df7c 100644
--- a/lib/RevBank/Cart.pm
+++ b/lib/RevBank/Cart.pm
@@ -21,6 +21,8 @@ sub add_entry($self, $entry) {
 
     push @{ $self->{entries} }, $entry;
     $self->{changed}++;
+    $self->select($entry);
+
     RevBank::Plugins::call_hooks("added_entry", $self, $entry);
 
     return $entry;
@@ -35,9 +37,7 @@ sub add($self, $amount, $description, $data = {}) {
     # ->add($user, ...)  => use $cart->add(...)->add_contra($user, ...)
     # ->add($entry)      => use $cart->add_entry($entry)
 
-    my $entry = $self->add_entry(RevBank::Cart::Entry->new($amount, $description, $data));
-    $self->select($entry);
-    return $entry;
+    return $self->add_entry(RevBank::Cart::Entry->new($amount, $description, $data));
 }
 
 sub select($self, $entry) {
diff --git a/plugins/deduplicate b/plugins/deduplicate
new file mode 100644
index 0000000..2fd042e
--- /dev/null
+++ b/plugins/deduplicate
@@ -0,0 +1,31 @@
+#!perl
+
+# Deduplication merges duplicate entries in the cart, e.g.
+# 3x cola + 4x cola = 7x cola.
+#
+# Plugins that support this, set the "deduplicate" attribute to a string key
+# that is used to determine which entries are equal. It is the responsibility
+# of the plugin that sets this, to ensure that the entries are indeed exactly
+# the same, if their deduplicate keys are equal.
+#
+# The recommended value for the deduplicate attribute is join("/", $plugin_id,
+# $unique_id), where $plugin_id can be obtained from $self->id in interactive
+# methods or $class->id in hooks. Including the plugin id avoids deduplicating
+# across plugins, that are probably not aware of eachothers $unique_id's.
+
+use List::Util qw(sum any);
+
+sub hook_added_entry($class, $cart, $added_entry, @) {
+    my $key = $added_entry->attribute('deduplicate') or return;
+
+    my @dedupe = grep {
+        $_->attribute('deduplicate') eq $key
+    } $cart->entries('deduplicate');
+
+    @dedupe >= 2 or return;
+
+    $dedupe[0]->quantity(sum map { $_->quantity } @dedupe);
+    $cart->select($dedupe[0]);
+
+    $cart->delete($_) for @dedupe[1 .. $#dedupe];
+}
diff --git a/plugins/market b/plugins/market
index 767d558..7fed980 100644
--- a/plugins/market
+++ b/plugins/market
@@ -35,21 +35,14 @@ sub command :Tab(market,&tab) ($self, $cart, $command, @) {
     my $space    = parse_amount($product->{ space })  or return NEXT;
     my $description = $product->{description};
 
-    my @existing = grep {
-        $_->attribute('plugin') eq $self->id and
-        $_->attribute('product_id') eq $command
-    } $cart->entries('plugin');
-
-    if (@existing) {
-        $existing[0]->quantity($existing[0]->quantity + 1);
-        $cart->select($existing[0]);
-        return ACCEPT;
-    }
-
     $cart->add(
         -($seller + $space),
         "$description (sold by $username)",
-        { product_id => $command, plugin => $self->id }
+        {
+            product_id => $command,
+            plugin => $self->id,
+            deduplicate => join("/", $self->id, $command),
+        }
     )->add_contra(
         $username,
         $seller,
diff --git a/plugins/products b/plugins/products
index 7fe1ae8..f172960 100644
--- a/plugins/products
+++ b/plugins/products
@@ -94,17 +94,6 @@ sub command :Tab(&tab) ($self, $cart, $command, @) {
     my $product = $products->{ $command } or return NEXT;
     my $price = $product->{price};
 
-    my @existing = grep {
-        $_->attribute('plugin') eq $self->id and
-        $_->attribute('product_id') eq $product->{id}
-    } $cart->entries('plugin');
-
-    if (@existing) {
-        $existing[0]->quantity($existing[0]->quantity + 1);
-        $cart->select($existing[0]);
-        return ACCEPT;
-    }
-
     my $contra_desc = "\$you bought $product->{description}";
 
     my @addons = @{ $product->{addons} // [] };
@@ -116,7 +105,12 @@ sub command :Tab(&tab) ($self, $cart, $command, @) {
     my $entry = $cart->add(
         -$price,
         $product->{description},
-        { product_id => $product->{id}, plugin => $self->id, product => $product }
+        {
+            product_id => $product->{id},
+            plugin => $self->id,
+            product => $product,
+            deduplicate => join("/", $self->id, $product->{id}),
+        }
     );
     $entry->add_contra(
         $product->{contra},
diff --git a/plugins/revspace_barcode b/plugins/revspace_barcode
index be2b981..81d1ccd 100644
--- a/plugins/revspace_barcode
+++ b/plugins/revspace_barcode
@@ -20,7 +20,11 @@ sub data($self, $cart, $input, @) {
         ->add(
             -$price,
             "Barcode <$input>",
-            { is_barcode => 1, barcode_data => $input }
+            {
+                is_barcode => 1,
+                barcode_data => $input,
+                deduplicate => join("/", $self->id, $input),
+            }
         )
         ->add_contra(
             "+sales/barcodes",
diff --git a/plugins/statiegeld b/plugins/statiegeld
index 7fd8cc8..d03483d 100644
--- a/plugins/statiegeld
+++ b/plugins/statiegeld
@@ -76,20 +76,12 @@ sub command { # args via @_ for mutable alias
 			? "$addon->{description}"
 			: "$addon->{description} ($product->{description})";
 
-		my @existing = grep {
-			$_->attribute('plugin') eq $invocant->id and
-			$_->attribute('addon_id') eq $addon->{id} and
-			$_->{description} eq $d
-		} $cart->entries('plugin');
-
-		if (@existing) {
-			$existing[0]->quantity($existing[0]->quantity + 1);
-			$cart->select($existing[0]);
-			next;
-		}
-
 		$cart
-		->add(+$addon->{price}, $d, { plugin => $invocant->id, addon_id => $addon->{id} })
+		->add(+$addon->{price}, $d, {
+			plugin => $invocant->id,
+			addon_id => $addon->{id},
+			deduplicate => join("/", $invocant->id, $addon->{id}),
+		})
 		->add_contra($addon->{contra}, -$addon->{price}, "$d for \$you");
 	}
 
diff --git a/revbank b/revbank
index 1a4c4a5..0d664cd 100755
--- a/revbank
+++ b/revbank
@@ -18,7 +18,7 @@ use RevBank::Global;
 use RevBank::Messages;
 use RevBank::Cart;
 
-our $VERSION = "3.7";
+our $VERSION = "3.8";
 our %HELP1 = (
     "abort" => "Abort the current transaction",
 );
diff --git a/revbank.plugins b/revbank.plugins
index 927549e..0136215 100644
--- a/revbank.plugins
+++ b/revbank.plugins
@@ -27,6 +27,7 @@ dinnerbonus
 # Then, plugins that apply heuristics
 
 repeat
+deduplicate  # wants to be after 'repeat'
 statiegeld
 statiegeld_tokens
 products   # matches product IDs (barcodes)