Do I still add sage?

I guess the answer to you is that we really can't give you one perfect answer.

The recipe is sort of vague. It calls for "sage" rather than "dried sage" or "fresh sage" and it doesn't specify a brand of "Italian seasoning", so there's no way to know whether or not theirs included sage already.

So, I'd ask myself a couple of questions:

1. How far down the ingredients list is sage in my Italian seasoning.

In a list of, say six ingredients, is it second or last? Or, are there twelve ingredients and it's tenth? Remember that ingredients lists are required to be listed from highest percentage to lowest. If it's high up the list, I'd be tempted to omit the sage. If it's really low down on a relatively long list, I'd guess that there's very little actual sage in the seasoning and go ahead and use the full amount in the recipe. If it's in the middle, use half.

2. How much do I like sage?

Sage can be a very strong ingredient, so it's understandable if you don't like it. If that's the case, I'd be more likely to leave it out. However, if you like sage, use it.

If I had to guess, sage is going to be pretty low on your ingredients list, so my general recommendation based on the information I have is that you use somewhere between half and all of the sage as listed in the recipe's ingredients and hope they mean dried sage because if they mean fresh, it's going to be really strong (dried herbs are much stronger than fresh herbs).

If you really aren't a fan of sage, leave it out entirely! It's not going to hurt anything one bit.

I would add the sage as well. The recipe calls for 3/4 tsp of italian seasoning and 1/2 tsp sage, and even if sage is 1/5 of the italian seasoning that would mean you are only adding 3/20 of a teaspoon of sage to the recipe with it, which isn't very much. By having sage as an extra ingredient the recipe is stating that sage needs to be prominent.

I have to say, calls for ingredients as vague as "Italian seasoning" in a recipe are a pet peeve of mine, for basically this reason.

One thing you could probably do in this case, since I see that the recipe you linked in your comment was updated recently, is contact the author and find out which Italian seasoning they used, if you wanted to be absolutely precise.

Otherwise, if you're in a rush, I say go for it and add the sage. It doesn't seem to me like so much sage or Italian seasoning that it's going to be a make-or-break difference one way or the other.

Two other tips that may help:

• Test for seasoning as you cook! You can definitely do that in this case. Try adding everything but the sage, tasting, and see what you think. Maybe you decide not to go with the extra sage if it tastes delicious as-is. Or add the sage and taste again! You might even decide that the 1/2 t sage isn't enough! I'm generally conservative my first time through a recipe, but I will often be more liberal about throwing in more or even different spices in a later go if I like it.
• For recipes you decide to keep, treat it as a science and keep a record of your experiments! I make notes on recipes that may include which brands/recipes I used for seasoning blends, what size cans I used, how many onions I bought at my local market to get the amount the recipe calls for, when I make substitutions, and when I decide to go for more/less spice, etc. It won't help you the first time, but it certainly helps for down the road.