User Secrets broke on different first word capitalization

### Describe the bug
On adding secret key with different first word capitalization, CLI tool fall with unhandled exception until manually fixing secret.json, or changing key Id at csproj

### To Reproduce
Steps to reproduce the behavior:
1. Execute at project DIR commands:
```bash
dotnet user-secrets set "ConnectionStrings:DefaultConnection" "test"
dotnet user-secrets set "connectionStrings:DefaultConnection1" "test"
```
2. Try to run anything else from user-secrets commands, for example:
```bash
dotnet user-secrets list
```
3. See error
```
System.ArgumentException: An item with the same key has already been added. Key: ConnectionStrings:DefaultConnection1
at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityCompa
rer`1 comparer)
at Microsoft.Extensions.SecretManager.Tools.Internal.SecretsStore.Load(String userSecretsId) in /_/src/dotnet-user-secrets/Internal/SecretsSto
re.cs:line 79
at Microsoft.Extensions.SecretManager.Tools.Internal.SecretsStore..ctor(String userSecretsId, IReporter reporter) in /_/src/dotnet-user-secret
s/Internal/SecretsStore.cs:line 32
at Microsoft.Extensions.SecretManager.Tools.Program.RunInternal(String[] args) in /_/src/dotnet-user-secrets/Program.cs:line 86
at Microsoft.Extensions.SecretManager.Tools.Program.TryRun(String[] args, Int32& returnCode) in /_/src/dotnet-user-secrets/Program.cs:line 36
Command failed : An item with the same key has already been added. Key: ConnectionStrings:DefaultConnection1
```

### Expected behavior
Show error on adding, or fixing that issue silently for user

### Additional context
```
.NET Core SDK (reflecting any global.json):
Version: 2.2.105
Commit: 7cecb35b92

Runtime Environment:
OS Name: Windows
OS Version: 10.0.14393
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\2.2.105\

Host (useful for support):
Version: 2.2.3
Commit: 6b8ad509b6

.NET Core SDKs installed:
2.1.403 [C:\Program Files\dotnet\sdk]
2.1.500 [C:\Program Files\dotnet\sdk]
2.2.105 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download
```