diff --git a/CHANGELOG.md b/CHANGELOG.md
index 899faf14..dd6e685a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@ Behaviour changes:
  * transactions management refactoring implied library API changes (#164)
  * add a separate `DBFT` generic parameter for transaction slice type
    specificatoin (#165)
+ * transaction duplicates in the proposal are not supported any more (#164, #167)
 
 Improvements:
  * minimum required Go version is 1.26 (#144, #156, #162)
diff --git a/config.go b/config.go
index 3947786b..545498f4 100644
--- a/config.go
+++ b/config.go
@@ -52,8 +52,9 @@ type Config[H Hash, Tx Transaction[H]] struct {
 	// StopTxFlow is a callback which is called when the process no longer needs
 	// any transactions.
 	StopTxFlow func()
-	// GetVerified returns a slice of verified transactions
-	// to be proposed in a new block.
+	// GetVerified returns a slice of verified transactions to be proposed in a
+	// new block. It's a caller's responsibility to check for the duplicates in
+	// the proposed set of transactions.
 	GetVerified func() []Tx
 	// VerifyPreBlock verifies if preBlock is valid.
 	VerifyPreBlock func(b PreBlock[H, Tx]) bool
diff --git a/context.go b/context.go
index 56ccdc9f..c1c6e064 100644
--- a/context.go
+++ b/context.go
@@ -56,6 +56,7 @@ type Context[H Hash, Tx Transaction[H]] struct {
 	Timestamp uint64
 	Nonce     uint64
 	// Transactions is a slice containing actual transactions for the current block.
+	// Duplicates are no-op and will lead to improper block construction.
 	Transactions []Tx
 	// MissingTransactions is a map of a missing transaction hash to its index in
 	// the slice of proposed transactions for the current block. This map is
diff --git a/prepare_request.go b/prepare_request.go
index 8870df4a..d6aa28c4 100644
--- a/prepare_request.go
+++ b/prepare_request.go
@@ -9,5 +9,7 @@ type PrepareRequest[H Hash, Tx Transaction[H]] interface {
 	// Transactions returns the list of all transactions in a proposed block
 	// with possible gaps in place of missing transactions and the map of
 	// missing transaction hashes to their indexes in the proposal list.
+	// Transaction duplicates are no-op and will lead to improper block
+	// construction.
 	Transactions() ([]Tx, map[H]int)
 }