V 1i is in post 1. Simon has been sleuthing again. Thanks Simon; you have been enormously helpful. 
Simon pointed out that OnlyTradeCurrencyTwice could not work as coded - and in fact would prevent an individual currency from being both bought and sold.
Here is my revised function so that DIYers can copy it over the top of the existing function. I have left the bad code commented out so you can see how it stopped the function working properly:
Code: Select all bool IsThisPairTradable(string symbol)
{
//Checks to see if either of the currencies in the pair is already being traded twice.
//If not, then return true to show that the pair can be traded, else return false
string c1 = StringSubstrOld(symbol, 0, 3);//First currency in the pair
string c2 = StringSubstrOld(symbol, 3, 3);//Second currency in the pair
int c1open = 0, c2open = 0;
CanTradeThisPair = true;
for (int cc = OrdersTotal() - 1; cc >= 0; cc--)
{
if (!BetterOrderSelect(cc, SELECT_BY_POS) ) continue;
////////if (OrderSymbol() != symbol ) continue;
if (OrderSymbol() == symbol ) continue;//We can allow multiple trades on the same symbol
if (OrderMagicNumber() != MagicNumber) continue;
int index = StringFind(OrderSymbol(), c1);
if (index > -1)
{
c1open++;
}//if (index > -1)
index = StringFind(OrderSymbol(), c2);
if (index > -1)
{
c2open++;
}//if (index > -1)
////////if (c1open == 1 && c2open == 1)
if (c1open > 1 || c2open > 1)
{
CanTradeThisPair = false;
return(false);
}//if (c1open > 1 || c2open > 1)
}//for (int cc = OrdersTotal() - 1; cc >= 0; cc--)
//Got this far, so ok to trade
return(true);
}//End bool IsThisPairTradable()
if (type == OP_BUY || type == OP_BUYSTOP)
This will cause the 'else' code block that examines sell trades to treat buy limit trades as sells - doesn't matter here as we do not deal with limit orders but correct code is always better than incorrect code. These two lines of code should be:
if (type == OP_BUY || type == OP_BUYSTOP || type == OP_BUYLIMIT)
All of this is legacy code that has been around since the dawn of time and so will be present in every EA I have coded and all my shells. This is something for you codeslingers using my shells to correct.
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Noobs the point behind all this is: we do not want to become over-exposed to a single currency and risk our accounts if something unexpected happens. For example, on Friday I had buys open on three EURxxx pairs. If a 'black swan' event had occurred causing the Euro to tank, then all those buys would have turned into losers within seconds and probably wiped the account. Zeljko and OnlyTradeCurrencyTwice are sisters in arms against over-trading.
Our Zeljko filter would have allowed only one EURxxx buy - EURUSD buy for example - and prevented CP from buying any of the others. It would have allowed CP to sell EURxxx - EURCHF for example.
So now we have an EU buy and an EChf sell open. The black swan flaps its wings and EU screams towards Hell like a guided missile on a mission. At the same time, EChf screams towards the heavens like an angel with good news to convey. The position is balanced and the account is safe.
Now imagine that we have a CHFJPY buy trade open at the same time. This is where OnlyTradeCurrencyTwice steps in to prevent over exposure to an individual currency. We have an EU buy, and EChr sell and a CHFJPY buy. CP spots an opportunity to sell EURCHF but examines the existing trades first, spots that we are already exposed the the Swissy twice and prevents the EC trade from going ahead and increasing our risk.
None of this helps the tiny demo account I am trying to build if something happens to either the US or Canadian dollar. I have three UCad buys open and heading hot-foot towards closing at the peak high. A dramatic tank right now would probably blow the account. Trading such a tiny account (start balance $300, current balance $340) is stupidly risky so I repeat my mantra: never trade with funds you cannot afford to lose.
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:youknow: 
Better keep a good eye on slippage.