diff --git a/coding/file_container.cpp b/coding/file_container.cpp
index 6af4112311..28716334e1 100644
--- a/coding/file_container.cpp
+++ b/coding/file_container.cpp
@@ -22,6 +22,13 @@ template <class TSink> void Write(TSink & sink, FilesContainerBase::Info const &
   WriteVarUint(sink, i.m_size);
 }
 
+string DebugPrint(FilesContainerBase::Info const & info)
+{
+  ostringstream ss;
+  ss << "{ " << info.m_tag << ", " << info.m_offset << ", " << info.m_size << " }";
+  return ss.str();
+}
+
 /////////////////////////////////////////////////////////////////////////////
 // FilesContainerBase
 /////////////////////////////////////////////////////////////////////////////
@@ -95,17 +102,23 @@ void FilesContainerW::Open(FileWriter::Op op)
 
   case FileWriter::OP_WRITE_EXISTING:
     {
-      {
-        // read an existing service info
-        FileReader reader(m_name);
-        ReadInfo(reader);
-      }
-
-      // Important: in append mode we should sort info-vector by offsets
-      sort(m_info.begin(), m_info.end(), LessOffset());
-      break;
+      // read an existing service info
+      FileReader reader(m_name);
+      ReadInfo(reader);
     }
 
+    // Important: in append mode we should sort info-vector by offsets
+    sort(m_info.begin(), m_info.end(), LessOffset());
+
+    // Check that all offsets are unique
+#ifdef DEBUG
+    for (size_t i = 1; i < m_info.size(); ++i)
+      ASSERT(m_info[i-1].m_offset < m_info[i].m_offset ||
+             m_info[i-1].m_size == 0 ||
+             m_info[i].m_size == 0, ());
+#endif
+    break;
+
   default:
     ASSERT ( false, ("Unsupported options") );
     break;
diff --git a/coding/file_container.hpp b/coding/file_container.hpp
index ad7f64af80..715023d0c9 100644
--- a/coding/file_container.hpp
+++ b/coding/file_container.hpp
@@ -5,6 +5,7 @@
 #include "../std/vector.hpp"
 #include "../std/string.hpp"
 
+
 class FilesContainerBase
 {
 public:
@@ -18,6 +19,8 @@ public:
 
     Info() {}
     Info(Tag const & tag, uint64_t offset) : m_tag(tag), m_offset(offset) {}
+
+    friend string DebugPrint(Info const & info);
   };
 
 protected:
@@ -36,19 +39,12 @@ protected:
       return (t1 < t2.m_tag);
     }
   };
+
   struct LessOffset
   {
     bool operator() (Info const & t1, Info const & t2) const
     {
-      if (t1.m_offset == t2.m_offset)
-      {
-        // Element with nonzero size should be the last one,
-        // for correct append writer mode (FilesContainerW::GetWriter).
-        ASSERT ( t1.m_size == 0 || t2.m_size == 0, (t1.m_size, t2.m_size) );
-        return (t1.m_size < t2.m_size);
-      }
-      else
-        return (t1.m_offset < t2.m_offset);
+      return (t1.m_offset < t2.m_offset);
     }
     bool operator() (Info const & t1, uint64_t const & t2) const
     {
@@ -59,6 +55,7 @@ protected:
       return (t1 < t2.m_offset);
     }
   };
+
   class EqualTag
   {
     Tag const & m_tag;