diff --git a/src/wallet/db.h b/src/wallet/db.h
--- a/src/wallet/db.h
+++ b/src/wallet/db.h
@@ -20,11 +20,17 @@
 #include <map>
 #include <memory>
 #include <string>
+#include <unordered_map>
 #include <vector>
 
 static const unsigned int DEFAULT_WALLET_DBLOGSIZE = 100;
 static const bool DEFAULT_WALLET_PRIVDB = true;
 
+struct WalletDatabaseFileId {
+    u_int8_t value[DB_FILE_ID_LEN];
+    bool operator==(const WalletDatabaseFileId &rhs) const;
+};
+
 class BerkeleyEnvironment {
 private:
     bool fDbEnvInit;
@@ -38,6 +44,7 @@
     std::unique_ptr<DbEnv> dbenv;
     std::map<std::string, int> mapFileUseCount;
     std::map<std::string, Db *> mapDb;
+    std::unordered_map<std::string, WalletDatabaseFileId> m_fileids;
     std::condition_variable_any m_db_in_use;
 
     BerkeleyEnvironment(const fs::path &env_directory);
diff --git a/src/wallet/db.cpp b/src/wallet/db.cpp
--- a/src/wallet/db.cpp
+++ b/src/wallet/db.cpp
@@ -31,32 +31,28 @@
 //! so bitcoin should never create different databases with the same fileid, but
 //! this error can be triggered if users manually copy database files.
 void CheckUniqueFileid(const BerkeleyEnvironment &env,
-                       const std::string &filename, Db &db) {
+                       const std::string &filename, Db &db,
+                       WalletDatabaseFileId &fileid) {
     if (env.IsMock()) {
         return;
     }
 
-    u_int8_t fileid[DB_FILE_ID_LEN];
-    int ret = db.get_mpf()->get_fileid(fileid);
+    int ret = db.get_mpf()->get_fileid(fileid.value);
     if (ret != 0) {
         throw std::runtime_error(strprintf(
             "BerkeleyBatch: Can't open database %s (get_fileid failed with %d)",
             filename, ret));
     }
 
-    for (const auto &item : env.mapDb) {
-        u_int8_t item_fileid[DB_FILE_ID_LEN];
-        if (item.second &&
-            item.second->get_mpf()->get_fileid(item_fileid) == 0 &&
-            memcmp(fileid, item_fileid, sizeof(fileid)) == 0) {
-            const char *item_filename = nullptr;
-            item.second->get_dbname(&item_filename, nullptr);
+    for (const auto &item : env.m_fileids) {
+        if (fileid == item.second && &fileid != &item.second) {
             throw std::runtime_error(strprintf(
                 "BerkeleyBatch: Can't open database %s (duplicates fileid %s "
                 "from %s)",
                 filename,
-                HexStr(std::begin(item_fileid), std::end(item_fileid)),
-                item_filename ? item_filename : "(unknown database)"));
+                HexStr(std::begin(item.second.value),
+                       std::end(item.second.value)),
+                item.first));
         }
     }
 }
@@ -67,6 +63,10 @@
 std::map<std::string, BerkeleyEnvironment> g_dbenvs;
 } // namespace
 
+bool WalletDatabaseFileId::operator==(const WalletDatabaseFileId &rhs) const {
+    return memcmp(value, &rhs.value, sizeof(value)) == 0;
+}
+
 BerkeleyEnvironment *GetWalletEnv(const fs::path &wallet_path,
                                   std::string &database_filename) {
     fs::path env_directory;
@@ -555,7 +555,8 @@
             // versions of BDB have an set_lk_exclusive method for this
             // purpose, but the older version we use does not.)
             for (const auto &dbenv : g_dbenvs) {
-                CheckUniqueFileid(dbenv.second, strFilename, *pdb_temp);
+                CheckUniqueFileid(dbenv.second, strFilename, *pdb_temp,
+                                  this->env->m_fileids[strFilename]);
             }
 
             pdb = pdb_temp.release();
@@ -909,6 +910,14 @@
             LOCK(cs_db);
             g_dbenvs.erase(env->Directory().string());
             env = nullptr;
+        } else {
+            // TODO: To avoid g_dbenvs.erase erasing the environment prematurely
+            // after the first database shutdown when multiple databases are
+            // open in the same environment, should replace raw database `env`
+            // pointers with shared or weak pointers, or else separate the
+            // database and environment shutdowns so environments can be shut
+            // down after databases.
+            env->m_fileids.erase(strFile);
         }
     }
 }
diff --git a/test/functional/wallet_multiwallet.py b/test/functional/wallet_multiwallet.py
--- a/test/functional/wallet_multiwallet.py
+++ b/test/functional/wallet_multiwallet.py
@@ -225,6 +225,11 @@
         assert_raises_rpc_error(-1, "BerkeleyBatch: Can't open database w8_copy (duplicates fileid",
                                 self.nodes[0].loadwallet, 'w8_copy')
 
+        # Fail to load if one wallet is a copy of another.
+        # Test this twice to make sure that we don't re-introduce https://github.com/bitcoin/bitcoin/issues/14304
+        assert_raises_rpc_error(-1, "BerkeleyBatch: Can't open database w8_copy (duplicates fileid",
+                                self.nodes[0].loadwallet, 'w8_copy')
+
         # Fail to load if wallet file is a symlink
         assert_raises_rpc_error(-4, "Wallet file verification failed: Invalid -wallet path 'w8_symlink'",
                                 self.nodes[0].loadwallet, 'w8_symlink')