Custom Resolution can't be added Mode already exists

[pommes_fritz]

If I go to display > general > resolution there's a plus symbol next to the actual value.
When I add now the recommended resolution of 2560x1440 I get the message: mode already exists (sorry, the screenshot shows german language), but it acutally don't. 

I'm using a corporate notebook (HP Elitebook x360 830 G7) and connected my private LG UltraGear 27GR95QE via Display Port to the HP Docking Station G5 which is connected via USB-C.
The display looks a little blurry, but everything is fine when I use the HDMI ports on my private pc (microsoft surface with docking station via mini-DP adaptor) or my PS5. 

So first thing I tried was to connect the monitor with hdmi direct to the notebook and everything worked fine. But then only this monitor is usable and, of course, I need the dock for the perephery and my second (corporate standard HP 23 inch) monitor that is connected.

On the docking station I had both monitors connected via DP, so I tried to connect the LG via HDMI, but with the same result.

After a little research in the web I tried to add a custom solution in the intel graphics app. I found out, that there was a CustomModeApp.exe in the windows sytem32 directory to activate it, but in newer versions it seems to be already built in:

@ROBERT_U_Intel ROBERT_U_Intel

Employee 

‎09-09-2014 07:32 AM

Custom modes are integrated with Intel HD Graphics Control Panel on the latest drivers and a separate Custom Modes App is no longer needed. You can access Custom Resolutions by opening the Intel HD Graphics Control Panel, click the Display tile, click the Display dropdown and select Custom Resolutions. (Intel HD 4400, can't find CustomModeApp.exe in System32 )

Had anyone the same issue? How can I solve this?

[MUC]

LG UltraGear OLED 27GR95QE-B, 26.5"

2560 x 1440 @ 240 Hz 10-bit HDR
2 x HDMI 2.1
1 x DisplayPort 1.4
VESA Adaptive Sync

HP EliteBook x360 830 G7
10th Gen Comet Lake platform
2 x Thunderbolt 3 (DisplayPort 1.2 HBR2)

HP USB-C Dock G5
For hosts that support DisplayPort 1.2:
2 x (2560 x 1440 @ 60 Hz 8-bit) = 14.7 Gbps
1 x 4K @ 60 Hz 8-bit = 16 Gbps

I think you have a problem with the available bandwidth here. You don't have to add resolutions manually. The resolutions that are not possible due to the lack of video bandwidth are simply not available for you to choose from and therefore "already exist". What model is the 2nd monitor exactly? Please create a Report for Intel Graphics Drivers and attach the output file here.

What happens if you unplug the 2nd monitor?

[pommes_fritz]

Thanks for the detailed answer. I'll send the model of the second one later.
But I figured out a workaround that works for me (hopefully temporary): I connected a dock for travel (HP USB-C Mini Dock USB 3.0) and connected it to the second USB-C of the notebook (awfully it only has one HDMI port).

It seems the G5 does not support more than 1080p.
Even if I only connect the LG higher resolution was not supported.

A report via the Intel desktop app was not possible, maybe the company blocks the connection.

[n_scott_pearson · Accepted solution]

Let's look at this another way. You have an embedded DisplayPort channel that can support DisplayPort 1.2 (i.e. 4K@60Hz). But, if the connected device also supports USB devices - as would be the case with any dock - then only half of the USB-C bandwidth will be available for use by DisplayPort (i.e. you are down to 4K@30Hz). 

...S

[pommes_fritz]

You are right. It's not enough bandwidth