diff --git a/db/src/db.rs b/db/src/db.rs
index 33289878..7df3138e 100644
--- a/db/src/db.rs
+++ b/db/src/db.rs
@@ -1344,18 +1344,17 @@ mod tests {
         assert_transact!(conn, "[[:db/add :db/tx :db/txInstant #inst \"2017-06-16T00:59:11.257Z\"]
                                  [:db/add :db/tx :db/txInstant #inst \"2017-06-16T00:59:11.752Z\"]
                                  [:db/add 102 :db/ident :name/Vlad]]",
-                         Err("Could not insert non-fts one statements into temporary search table!"));
+                         Err("conflicting datoms in tx"));
 
         // Test multiple txInstants with the same value.
-        // Test disabled: depends on #535.
-        // assert_transact!(conn, "[[:db/add :db/tx :db/txInstant #inst \"2017-06-16T00:59:11.257Z\"]
-        //                          [:db/add :db/tx :db/txInstant #inst \"2017-06-16T00:59:11.257Z\"]
-        //                          [:db/add 103 :db/ident :name/Dimitri]
-        //                          [:db/add 104 :db/ident :name/Anton]]");
-        // assert_matches!(conn.last_transaction(),
-        //                 "[[103 :db/ident :name/Dimitri ?tx true]
-        //                   [104 :db/ident :name/Anton ?tx true]
-        //                   [?tx :db/txInstant #inst \"2017-06-16T00:59:11.257Z\" ?tx true]]");
+        assert_transact!(conn, "[[:db/add :db/tx :db/txInstant #inst \"2017-06-16T00:59:11.257Z\"]
+                                 [:db/add :db/tx :db/txInstant #inst \"2017-06-16T00:59:11.257Z\"]
+                                 [:db/add 103 :db/ident :name/Dimitri]
+                                 [:db/add 104 :db/ident :name/Anton]]");
+        assert_matches!(conn.last_transaction(),
+                        "[[103 :db/ident :name/Dimitri ?tx true]
+                          [104 :db/ident :name/Anton ?tx true]
+                          [?tx :db/txInstant #inst \"2017-06-16T00:59:11.257Z\" ?tx true]]");
 
         // Test txInstant retraction
         // Test disabled: retracting a datom that doesn't exist should fail.
diff --git a/db/src/errors.rs b/db/src/errors.rs
index 62a263d0..15b8614c 100644
--- a/db/src/errors.rs
+++ b/db/src/errors.rs
@@ -82,5 +82,10 @@ error_chain! {
             description("unrecognized or no ident found for entid")
             display("unrecognized or no ident found for entid: {}", entid)
         }
+
+        ConflictingDatoms {
+            description("conflicting datoms in tx")
+            display("conflicting datoms in tx")
+        }
     }
 }
diff --git a/db/src/lib.rs b/db/src/lib.rs
index f9416319..2c9ca700 100644
--- a/db/src/lib.rs
+++ b/db/src/lib.rs
@@ -8,6 +8,9 @@
 // CONDITIONS OF ANY KIND, either express or implied. See the License for the
 // specific language governing permissions and limitations under the License.
 
+// Oh, error_chain.
+#![recursion_limit="128"]
+
 #[macro_use]
 extern crate error_chain;
 extern crate itertools;
diff --git a/db/src/tx.rs b/db/src/tx.rs
index 45d11e6d..8adac2a5 100644
--- a/db/src/tx.rs
+++ b/db/src/tx.rs
@@ -633,11 +633,21 @@ impl<'conn, 'a> Tx<'conn, 'a> {
                     let added = op == OpType::Add;
 
                     // We take the last encountered :db/txInstant value.
+                    // If more than one is provided, the transactor will fail.
                     if added &&
                        e == self.tx_id &&
                        a == entids::DB_TX_INSTANT {
                         if let TypedValue::Instant(instant) = v {
-                            self.tx_instant = Some(instant);
+                            if let Some(ts) = self.tx_instant {
+                                if ts == instant {
+                                    // Dupes are fine.
+                                } else {
+                                    bail!(ErrorKind::ConflictingDatoms);
+                                }
+                            } else {
+                                self.tx_instant = Some(instant);
+                            }
+                            continue;
                         } else {
                             // The type error has been caught earlier.
                             unreachable!()
@@ -657,14 +667,12 @@ impl<'conn, 'a> Tx<'conn, 'a> {
 
         tx_instant = self.tx_instant.unwrap_or_else(now);
 
-        // Transact [:db/add :db/txInstant NOW :db/tx] if it doesn't exist.
-        if self.tx_instant == None {
-            non_fts_one.push((self.tx_id,
-                              entids::DB_TX_INSTANT,
-                              self.schema.require_attribute_for_entid(entids::DB_TX_INSTANT).unwrap(),
-                              TypedValue::Instant(tx_instant),
-                              true));
-        }
+        // Transact [:db/add :db/txInstant tx_instant :db/tx].
+        non_fts_one.push((self.tx_id,
+                          entids::DB_TX_INSTANT,
+                          self.schema.require_attribute_for_entid(entids::DB_TX_INSTANT).unwrap(),
+                          tx_instant.into(),
+                          true));
 
         if !non_fts_one.is_empty() {
             self.store.insert_non_fts_searches(&non_fts_one[..], db::SearchType::Inexact)?;