No duplicate addresses on different chains

EIP-no-duplicate-addresses-on-different-chains.md

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+## Preamble
+
+    EIP: <to be assigned>
+    Title: No duplicate addresses on different chains
+    Author: James Ray
+    Type: Standard Track
+    Category: Core
+    Status: Draft
+    Created: 2017-02-16
+    Requires: [EIP-155](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-155.md), [EIP-161](https://github.com/ethereum/EIPs/blob/master/EIPS/eip-161.md)
+
+## Simple Summary
+Implement one or more options to avoid someone sending to an address that is on the main network, but it was actually meant to be sent to the same address on a different network, or vice versa.
+
+## Abstract
+This EIP aims to avoid one particular case of lost funds where funds are sent to an address on one network, while the funds were actually meant to be sent to the same address, but on a different network. This case is described in detail [here](https://github.com/ethereum/EIPs/pull/867/files#diff-2c90d5bf27887dac3a3d5f6d048567d5R52] in EIP 867, and is replicated below for convenience:
+
+>*Considerable example** (concise, includes supporting evidence, no negative impact): 
+*A crowdsale run by XYZ incorrectly published the testnet address of their crowdsale contract to their public website at the start of their crowdsale on Jan 19, 2018.  501 ETH was sent by 328 users on the mainnet to the incorrect address between block 4,235,987 and 4,236,050.  See here for the testnet contract, and see here for the transactions to the same address on the mainnet.  See here for a statement made by XYZ on their website.  Because there is a contract at this address on the testnet and the corresponding nonce for the creator address has already been used on the mainnet, it is considered effectively impossible that anyone coincidentally holds the private key. We have verified that all transactions came from addresses with no associated code, so there should be no issue returning eth to the senders.*  
+
+
+## Motivation
+
+## Specification
+This is just an example to illustrate how it might work:
+```python
+% Vyper
+from enum import Enum
+
+def isNonExistentAccount(address _address):
+    if _address == "":
+        return true;
+
+% This is already done in EIP-161
+def isEmptyAccount(address _address):
+    if (_address.nonce == 0 && % pseudocode: nonce is not formally a member of the address type.
+                % For the main net this will be 0, but for other nets this may be different
+            _address.balance) == 0 &&
+            _address.codesize == 0):
+        % delete account
+        _address == ""
+        return "The address had an empty account but it is now deleted";
+    else:
+        return false;
+
+class chain_ID(Enum):
+    % See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-155.md#list-of-chain-ids.
+    ETHEREUM_MAINNET = 1
+    EXPANSE_MAINNET = 2 % Morden (disused), Expanse mainnet
+    ROPSTEN = 3
+    RINKEBY = 4
+    ROOTSTOCK_MAINNET = 30
+    ROOTSTOCK_TESTNET = 31
+    KOVAN = 42 % Kovan
+    ETHEREUM_CLASSIC_MAINNET = 61
+    ETHEREUM_CLASSIC_TESTNET = 62
+    GETH_PRIVATE_CHAINS_DEFAULT = 1337
+
+def isExistingAddressOnAChainID(chain_ID _chain_ID, address _address):
+    % Pseudocode
+    if not isNonExistentAccount(_chain_ID._address) || not isEmptyAccount(_chain_ID._address):
+        return = true;
+
+def isExistingAddressOnAnotherChainID(address _address)
+    for i in chain_ID:
+        if isExistingAddressOnAChainID(chain_ID[i], _address):
+            % I'm not sure if this is correct syntax, it's not clear from here: https://docs.python.org/3/library/enum.html?highlight=enum#module-enum:
+            for j in chain_ID[i+1:]:
+                if isExistingAddressOnAChainID(chain_ID[j], _address):
+                    return true;
+        else:
+            continue;
+    else:
+        return false;
+def makeATransaction(wei_value value, address: _to, chain_ID _chain_ID)
+    if isExistingAddressOnAnotherChainID(_to):
+        confirmTransaction = "The same address %d exists on at least 2 chain_IDs, are you sure that you want to send it to %d? Enter Y/N: " % (_to, _chain_ID)
+        if confirmTransaction == "N"
+            break;
+        elseif confirmTransaction == "Y":
+            % proceed with the transaction.
+        else:
+            print "You didn't enter Y/N, please try again."
+```
+
+## Rationale
+I haven't considered alternative designs, the above is just a minimum example of how it could be implemented.
+
+## Backwards Compatibility
+I don't think this introduces backwards incompatibilities.
+
+## Test Cases
+Not done.
+
+## Implementation
+The implementations must be completed before any EIP is given status "Final", but it need not be completed before the EIP is accepted. While there is merit to the approach of reaching consensus on the specification and rationale before writing code, the principle of "rough consensus and running code" is still useful when it comes to resolving many discussions of API details.
+
+## Copyright
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).