I entered an amount in capital gains, say it was $1000. Now in the summary TT claims I had $1000 in taxable cap gains. But that should be $500. What to do?

Do you also have t-slips (imported using Auto-Fill or manually entered by you) with capital gains reported?

I had just a single entry on my schedule 3. (Of course I had other T-slips, like a T-4 and a T-5, but this seems totally irrelevant). Again, the trouble is that it adds the cap gains, rather than the taxable cap gains to my income. It's a big difference

In TurboTax Online, when I enter capital gains of $1,000, my capital gains summary shows a Gain of 1,000.  When I go to the Income Summary, I see 500.00 next to Taxable capital gains.  Sometimes when customers are seeing double the amount in capital gains that they are expecting, it's because they imported slips with their capital gains transactions included, and then reported those same transactions again in the capital gains section.  Slips can include T3, T4PS, T5, T5008, or T5013.  If you are looking right at your Schedule 3, you can check to see if you have amounts flowing in from slips.

1. Where do I find my schedule 3 though? 2. Also, is it possible that this is the explanation if the capital gain itself is definitely not on a T slip (as it is from the sale of foreign shares held at a foreign broker)

Go to File > Print Your Tax Return.  You might need to complete payment if you haven't already, but you do not need to be finished with your return to be able to generate this PDF.  Schedule 3 should give you more insight into your taxable capital gains situation.

Hi Sarah. This is helpful. Indeed TT must have created a T-slips with exactly the same amount on it. I conclude this from "T5, T5013, and T4PS information slips – Capital gains (or losses)" which is followed by (another time) the capital gains amount. This brings me to the next question, how did TT create a T slip and (a) how can I view that T-slip, and (b) perhaps remove that T-slip? [again, it must be a T-slip that Turbotax somehow created. Thanks!

It seems it have been a T5008 that TT created

You can go to Income > T-Slips > T5008 to edit or delete the slip.  If you don't remember entering it, it may have been imported through Auto-Fill My Return.

From the TurboTax FAQ: 

The amount of capital gains from my T5008 is doubled in TurboTax. How do I fix this?