net: Prevent node from binding to the same CService

Currently, if the user inadvertently starts the node with duplicate bind options,
such as `-bind=0.0.0.0 -bind=0.0.0.0`, it will cause a fatal error with the
misleading message "Bitcoin Core is probably already running".

This commit adds early validation to detect duplicate bindings across all binding
configurations (-bind, -whitebind, and onion bindings) before attempting to bind.
When duplicates are detected, the node terminates with a clear, specific error
message: "Duplicate binding configuration for address <addr>. Please check your
-bind, -bind=...=onion and -whitebind settings."

The validation catches duplicates both within the same option type (e.g.,
`-bind=X -bind=X`) and across different types (e.g., `-bind=X -whitebind=Y@X`),
helping users identify and fix configuration mistakes.

src/init.cpp

@@ -1233,6 +1233,40 @@ bool CheckHostPortOptions(const ArgsManager& args) {
     return true;
 }
 
+/**
+ * @brief Checks for duplicate bindings across all binding configurations.
+ *
+ * @param[in] conn_options Connection options containing the binding vectors to check
+ * @return std::optional<CService> containing the first duplicate found, or std::nullopt if no duplicates
+ */
+static std::optional<CService> CheckBindingConflicts(const CConnman::Options& conn_options)
+{
+    std::set<CService> seen;
+
+    // Check all whitelisted bindings
+    for (const auto& wb : conn_options.vWhiteBinds) {
+        if (!seen.insert(wb.m_service).second) {
+            return wb.m_service;
+        }
+    }
+
+    // Check regular bindings
+    for (const auto& bind : conn_options.vBinds) {
+        if (!seen.insert(bind).second) {
+            return bind;
+        }
+    }
+
+    // Check onion bindings
+    for (const auto& onion_bind : conn_options.onion_binds) {
+        if (!seen.insert(onion_bind).second) {
+            return onion_bind;
+        }
+    }
+
+    return std::nullopt;
+}
+
 // A GUI user may opt to retry once with do_reindex set if there is a failure during chainstate initialization.
 // The function therefore has to support re-entry.
 static ChainstateLoadResult InitAndLoadChainstate(
@@ -2142,6 +2176,13 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
 
     connOptions.m_i2p_accept_incoming = args.GetBoolArg("-i2pacceptincoming", DEFAULT_I2P_ACCEPT_INCOMING);
 
+    if (auto conflict = CheckBindingConflicts(connOptions)) {
+        return InitError(strprintf(
+            _("Duplicate binding configuration for address %s. "
+                "Please check your -bind, -bind=...=onion and -whitebind settings."),
+                    conflict->ToStringAddrPort()));
+    }
+
     if (!node.connman->Start(scheduler, connOptions)) {
         return false;
     }