--- backup.py	2021-02-13 06:42:22.000000000 +0200
+++ backup_twice.py	2021-02-13 06:41:46.000000000 +0200
@@ -3,26 +3,27 @@
 import itertools
 
 storages=[700, 30, 100, 800, 100, 800, 300, 150, 60, 500, 1000]
-files=[18, 57, 291, 184, 167, 496, 45, 368, 144, 428, 15, 100, 999]
+files=[18, 57, 291, 184, 167, 496, 45, 368, 144, 15]
 
 files_n=len(files)
 files_t=sum(files)
 
-print ("total storage we need", files_t)
+print ("total storage we need", files_t*2)
 
 def try_to_fit_into_storages(storages_to_be_used):
     t=len(storages_to_be_used)
     # for each server:
     storage_occupied=[Int('storage%d_occupied' % i) for i in range(t)]
     # which storage the file occupies?
-    file_in_storage=[Int('file%d_in_storage' % i) for i in range(files_n)]
+    file1_in_storage=[Int('file1_%d_in_storage' % i) for i in range(files_n)]
+    file2_in_storage=[Int('file2_%d_in_storage' % i) for i in range(files_n)]
 
     # how much storage we have in picked storages, total?
     storage_t=0
     for i in range(t):
         storage_t=storage_t+storages[storages_to_be_used[i]]
     # skip if the sum of storage in picked storages is too small:
-    if files_t > storage_t:
+    if files_t*2 > storage_t:
         return
 
     print ("trying to fit all the files into storages:", storages_to_be_used,end=' ')
@@ -31,7 +32,10 @@
 
     # all files must occupy some storage:
     for i in range(files_n):
-        s.add(And(file_in_storage[i]>=0, file_in_storage[i]<t))
+        s.add(And(file1_in_storage[i]>=0, file1_in_storage[i]<t))
+        s.add(And(file2_in_storage[i]>=0, file2_in_storage[i]<t))
+        # no pair can reside on one storage:
+        s.add(file1_in_storage[i] != file2_in_storage[i])
 
     for i in range(t):
         """
@@ -52,7 +56,9 @@
 
         """
 
-        s.add(storage_occupied[i]==Sum([If(file_in_storage[f]==i, files[f], 0) for f in range(files_n)]))
+        s.add(storage_occupied[i]==
+            Sum([If(file1_in_storage[f]==i, files[f], 0) for f in range(files_n)])+
+            Sum([If(file2_in_storage[f]==i, files[f], 0) for f in range(files_n)]))
 
         # ... but sum of all files in each storage must be lower than what we have in the storage:
         s.add(And(storage_occupied[i]>=0, storage_occupied[i]<=storages[storages_to_be_used[i]]))
@@ -64,12 +70,14 @@
         for i in range(t):
             print ("storage%d (total=%d):" % (storages_to_be_used[i], storages[storages_to_be_used[i]]))
             for f in range(files_n):
-                if m[file_in_storage[f]].as_long()==i:
-                    print ("    file%d (%d)" % (f, files[f]))
+                if m[file1_in_storage[f]].as_long()==i:
+                    print ("    file%d (1st copy) (%d)" % (f, files[f]))
+                if m[file2_in_storage[f]].as_long()==i:
+                    print ("    file%d (2nd copy) (%d)" % (f, files[f]))
             print ("allocated on storage=%d" % (m[storage_occupied[i]].as_long()),end=' ')
             print ("free on storage=%d" % (storages[storages_to_be_used[i]] - m[storage_occupied[i]].as_long()))
         print ("total in all storages=%d" % storage_t)
-        print ("allocated on all storages=%d%%" % ((float(files_t)/float(storage_t))*100))
+        print ("allocated on all storages=%d%%" % ((float(files_t*2)/float(storage_t))*100))
         print ("")
         return True
     else: