diff --git a/Cargo.lock b/Cargo.lock
index dd6c5cc..0b54a75 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -26,6 +26,17 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
+[[package]]
+name = "getrandom"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7fcd999463524c52659517fe2cea98493cfe485d10565e7b0fb07dbba7ad2753"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "wasi",
+]
+
 [[package]]
 name = "libc"
 version = "0.2.109"
@@ -37,6 +48,7 @@ name = "lookaround"
 version = "0.1.0"
 dependencies = [
  "mac_address",
+ "rand",
  "thiserror",
 ]
 
@@ -72,6 +84,12 @@ dependencies = [
  "memoffset",
 ]
 
+[[package]]
+name = "ppv-lite86"
+version = "0.2.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ed0cfbc8191465bed66e1718596ee0b0b35d5ee1f41c5df2189d0fe8bde535ba"
+
 [[package]]
 name = "proc-macro2"
 version = "1.0.32"
@@ -90,6 +108,46 @@ dependencies = [
  "proc-macro2",
 ]
 
+[[package]]
+name = "rand"
+version = "0.8.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2e7573632e6454cf6b99d7aac4ccca54be06da05aca2ef7423d22d27d4d4bcd8"
+dependencies = [
+ "libc",
+ "rand_chacha",
+ "rand_core",
+ "rand_hc",
+]
+
+[[package]]
+name = "rand_chacha"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e6c10a63a0fa32252be49d21e7709d4d4baf8d231c2dbce1eaa8141b9b127d88"
+dependencies = [
+ "ppv-lite86",
+ "rand_core",
+]
+
+[[package]]
+name = "rand_core"
+version = "0.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d34f1408f55294453790c48b2f1ebbb1c5b4b7563eb1f418bcfcfdbb06ebb4e7"
+dependencies = [
+ "getrandom",
+]
+
+[[package]]
+name = "rand_hc"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d51e9f596de227fda2ea6c84607f5558e196eeaf43c986b724ba4fb8fdf497e7"
+dependencies = [
+ "rand_core",
+]
+
 [[package]]
 name = "syn"
 version = "1.0.82"
@@ -127,6 +185,12 @@ version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8ccb82d61f80a663efe1f787a51b16b5a51e3314d6ac365b08639f52387b33f3"
 
+[[package]]
+name = "wasi"
+version = "0.10.2+wasi-snapshot-preview1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fd6fbd9a79829dd1ad0cc20627bf1ed606756a7f77edff7b66b7064f9cb327c6"
+
 [[package]]
 name = "winapi"
 version = "0.3.9"
diff --git a/Cargo.toml b/Cargo.toml
index 133a946..1e8cd78 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,6 +7,7 @@ edition = "2021"
 
 [dependencies]
 mac_address = "1.1.2"
+rand = "0.8.4"
 thiserror = "1.0.30"
 
 [profile.release]
diff --git a/src/main.rs b/src/main.rs
index 9048a12..971d571 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -87,14 +87,26 @@ fn main () -> Result <(), AppError> {
 }
 
 fn client () -> Result <(), AppError> {
+	use rand::RngCore;
+	
 	let params = CommonParams::default ();
 	let socket = UdpSocket::bind ("0.0.0.0:0")?;
 	
 	socket.join_multicast_v4 (&params.multicast_addr, &([0u8, 0, 0, 0].into ()))?;
 	socket.set_read_timeout (Some (Duration::from_millis (1_000)))?;
 	
-	let msg = Message::Request (None).to_vec ()?;
-	socket.send_to (&msg, (params.multicast_addr, params.server_port))?;
+	let mut idem_id = [0u8; 8];
+	rand::thread_rng ().fill_bytes (&mut idem_id);
+	
+	let msg = Message::Request {
+		idem_id,
+		mac: None,
+	}.to_vec ()?;
+	
+	for _ in 0..5 {
+		socket.send_to (&msg, (params.multicast_addr, params.server_port))?;
+		std::thread::sleep (Duration::from_millis (50));
+	}
 	
 	let start_time = Instant::now ();
 	
@@ -139,13 +151,26 @@ fn server () -> Result <(), AppError> {
 	
 	socket.join_multicast_v4 (&params.multicast_addr, &([0u8, 0, 0, 0].into ())).unwrap ();
 	
+	let mut recent_idem_ids = Vec::with_capacity (32);
+	
 	loop {
 		println! ("Waiting for messages...");
 		let (req, remote_addr) = recv_msg_from (&socket)?;
 		
 		let resp = match req {
-			Message::Request (None) => {
-				Some (Message::Response (our_mac))
+			Message::Request {
+				mac: None,
+				idem_id,
+			} => {
+				if recent_idem_ids.contains (&idem_id) {
+					println! ("Ignoring request we already processed");
+					None
+				}
+				else {
+					recent_idem_ids.insert (0, idem_id);
+					recent_idem_ids.truncate (30);
+					Some (Message::Response (our_mac))
+				}
 			},
 			_ => continue,
 		};
diff --git a/src/message.rs b/src/message.rs
index 2626ad2..ad91c95 100644
--- a/src/message.rs
+++ b/src/message.rs
@@ -11,7 +11,10 @@ pub const PACKET_SIZE: usize = 1024;
 
 #[derive (Debug)]
 pub enum Message {
-	Request (Option <[u8; 6]>),
+	Request {
+		idem_id: [u8; 8],
+		mac: Option <[u8; 6]>
+	},
 	Response (Option <[u8; 6]>),
 }
 
@@ -31,8 +34,12 @@ impl Message {
 		w.write_all (&MAGIC_NUMBER)?;
 		
 		match self {
-			Self::Request (mac) => {
+			Self::Request {
+				idem_id,
+				mac,
+			}=> {
 				w.write_all (&[1])?;
+				w.write_all (&idem_id[..])?;
 				Self::write_mac_opt (w, *mac)?;
 			},
 			Self::Response (mac) => {
@@ -68,8 +75,14 @@ impl Message {
 		
 		Ok (match t {
 			1 => {
+				let mut idem_id = [0u8; 8];
+				r.read_exact (&mut idem_id)?;
+				
 				let mac = Self::read_mac_opt (r)?;
-				Self::Request (mac)
+				Self::Request {
+					idem_id,
+					mac,
+				}
 			},
 			2 => {
 				let mac = Self::read_mac_opt (r)?;