diff --git a/lib/blockcache/blockcache.go b/lib/blockcache/blockcache.go
index e3cc45465..62c479a85 100644
--- a/lib/blockcache/blockcache.go
+++ b/lib/blockcache/blockcache.go
@@ -109,20 +109,21 @@ func (c *Cache) cleaner() {
 }
 
 func (c *Cache) cleanByTimeout() {
-	currentTime := fasttime.UnixTimestamp()
+	// Delete items accessed more than five minutes ago.
+	// This time should be enough for repeated queries.
+	lastAccessTime := fasttime.UnixTimestamp()-5*60
 	c.mu.Lock()
+	defer c.mu.Unlock()
+
 	for _, pes := range c.m {
 		for offset, e := range pes {
-			// Delete items accessed more than five minutes ago.
-			// This time should be enough for repeated queries.
-			if currentTime-atomic.LoadUint64(&e.lastAccessTime) > 5*60 {
+			if lastAccessTime > atomic.LoadUint64(&e.lastAccessTime) {
 				c.updateSizeBytes(-e.block.SizeBytes())
 				delete(pes, offset)
 				// do not delete the entry from c.perKeyMisses, since it is removed by Cache.cleaner later.
 			}
 		}
 	}
-	c.mu.Unlock()
 }
 
 // GetBlock returns a block for the given key k from c.
@@ -167,14 +168,19 @@ func (c *Cache) PutBlock(k Key, b Block) {
 
 	// Store b in the cache.
 	c.mu.Lock()
-	e := &cacheEntry{
-		lastAccessTime: fasttime.UnixTimestamp(),
-		block:          b,
-	}
+	defer c.mu.Unlock()
+
 	pes := c.m[k.Part]
 	if pes == nil {
 		pes = make(map[uint64]*cacheEntry)
 		c.m[k.Part] = pes
+	} else if pes[k.Offset] != nil {
+		// The block has been already registered by concurrent goroutine.
+		return
+	}
+	e := &cacheEntry{
+		lastAccessTime: fasttime.UnixTimestamp(),
+		block:          b,
 	}
 	pes[k.Offset] = e
 	c.updateSizeBytes(e.block.SizeBytes())
@@ -187,13 +193,11 @@ func (c *Cache) PutBlock(k Key, b Block) {
 				delete(pes, offset)
 				// do not delete the entry from c.perKeyMisses, since it is removed by Cache.cleaner later.
 				if c.SizeBytes() < maxSizeBytes {
-					goto end
+					return
 				}
 			}
 		}
 	}
-end:
-	c.mu.Unlock()
 }
 
 // Len returns the number of blocks in the cache c.